some questions about miui and updateing..

[Shahar287]

hi, i'm new to miui and bout Xiaomi Mi4 a week ago:
1. what's the difference between the numbers of the versions of stable and developers?
for example, the stable is 6.2.1.0
and the developers is 4.12.19
why is the main numbers (6/4) are different from each other? what's the difference?
2. what's the difference between updating from recovery mode and updater? if i would like to get the developers version from the stable, i do wipe data, wipe cache and wipe all and then restarting the system, transfering the update file and then just update? from the updater app or from recovery?
4. what's the difference between flashing and installing rom?
5. after i'm updating to the weekly developers rom, i guess the updater app will get a notification that a new update release, should i update directly from the app?
6. i noticed that there's a difference in the files size of the versions downloads here in this site.. they're different from the official files from eng.xiaomi.com. for example, 4.12.19 file in this site is 391 MB and in eng.xiaomi.com it's 365 MB!!
thanks!