Can you pls explain me why so big change on the version number since the repository update version is small and the update not crucial?
There are many plexpass releases in between that you don’t have access to.
Because there was a lot of changes between 1.5.7 and 1.7.5 including plex web updates, a transcoder updates. You can’t just ignore the branch number when looking at the version numbers. They do post all the server version changes here.
1.6.0:
NEW:
(Web) Updated Plex Web to 3.4.1
(Localization) Updated server translations
(NAS) Synology x86_64 package support
(NAS) Use /shares as the library root for WD MyCloud (#6678)
FIXES:
(Playlists) ‘Play All’ may not play in the expected order (#5035 #5071)
(Playlists) Thumnails may not be generated for smart playlists (#6459)
(Search) Searches including an apostrophe would fail (#6585)
(DVR) Recording timeslot preference could be off by one hour during DST (#5671)
(DVR) End-padded recordings can terminate prematurely (#6284)
(Metadata) TMDB API errors could cause PlexMovie agent failures
(Network) Crashes when diagnosing with the connections endpoint (#5713)
(Linux) Ubuntu/Fedora/CentOS systemd startup warnings (#6741)
(Smart TV) Playback could fail when servers were not signed in (#6715)
(PMP) Playback could fail when servers were not signed in (#6524)
1.6.1
FIXES:
(Transcoder) Fix an issue with transcodes failing with long Windows user names. (#6709)
1.7.0
NEW:
(Transcoder) Updated transcoder
(Library) Enable the ‘Allow media deletion’ preference on new and default installations (#6753)
(NAS) Added Synology multivolume support (#6782)
(DVR) Recordings are now remuxed by default
FIXED:
(Camera Upload) Trying to upload files larger than 4GB would fail.
(Linux) Install issue if user has a systemd override.conf in place for PMS. (#6904)
(Linux) Install error on Ubuntu 17.04 (#6783)
(Music) Albums were incorrectly sorted (#6537 #6043)
(Streaming Brain) Rare deadlock when transcoding DASH
(Scannner) Cancelling a photo section scan could take a significant amount of time (#6663 #6699)
1.7.1
FIXED:
(Streaming Brain) Incorrectly chose to Direct Stream audio under certain circumstances
1.7.2
NEW:
(Web) Updated Plex Web to 3.7.0
FIXED:
(Metadata) Use 16x9 aspect ratios for 480p and 576p resolution limits. (#6795)
(MDE) Corrected codec decision to prefer AAC over MP3 when on windows (#6737)
(DVR) Stutter during playback of remuxed DVR recordings (#7070)
(Tags) Tags including numbers were sorted incorrectly (#7027)
(DVR) Multiple concurrent recordings could not be made with the HDHR Dual (#6934)
1.7.3
NEW:
(DVR) Support of extra versions of the WinTV QuadHD
FIXED:
(Linux) RPM package would overwrite repo files on upgrades. (#7078)
(Chromecast) Multi-channel audio would not play back on updated devices (#7089)
(Companion) Remote control could be unreliable in some cases (e.g. Xbox One) (#3635)
(Sync) Shared users would not see sync updates in the web UI
(Hubs) Disabled upcoming movie trailer hub (#7099)
1.7.4
NEW:
(Web) Updated Plex Web to 3.9.1
(DVR) Support for WinTV-HVR-22XX PCIe tuners
(DVR) Support for timeshift in live streams (requires upcoming iOS v4.14 and Android TV v6.2 app releases)
(Butler) Allow server updates triggered from client apps to occur during nightly maintenance (#6246)
FIXED:
(Windows) Failures when transcoding with EAE
(Windows) Cygwin1.dll reported as insecure by Secunia PSI, updated to 2.8.0
(Windows) Failures transcoding AAC on older versions of Windows when using the Media Foundation AAC Encoder (#7132)
(Butler) Clean up previous updaters during maintenance. PMS will keep the previous three updates (#4510)
(Photos) Moving files between folders inside the same section may result in the timeline view becoming inconsistent.
(Xbox 360) Videos with MP3 audio could go out-of-sync. (#6989)
(Discovery) Improve quality of related media on movie/show pages.
(DVR) Under certain circumstances, duplicate recordings could occur (#7144)
(DVR) Some supported WinTV models were not detected
(DVR) TVButler was not detected on Ubuntu/CentOS/Fedora/Synology (#7162)
(SHEILD) Transcoded videos were incorrectly cropped
1.7.5
FIXED:
(Linux) Ubuntu package would not install unless udev was installed (#7205)
Understood! Thank you for your replies 
