HTC has been able to shoe horn the entire Android 4.4.4 update into Android 4.4.3. That means that versions of the flagship phone receiving Android 4.4.3 will then go straight to Android L. With the international update rolling out, it won't be terribly long before AT&T, T-Mobile and Verizon customers receive the same update. Don't forget that HTC has promised to roll out Android L to HTC One (M8) and HTC One (M7) owners, no later than 90 days after it receives the final update from Google.
So to recap, the Google Play edition of the HTC One (M8) has received Android 4.4.4. Sprint customers have received a version of Android 4.4.3 that combines both Android 4.4.3 and Android 4.4.4. That update is being pushed out this week to unlocked international versions of the phone, and will soon go out to AT&T, T-Mobile and Verizon branded HTC One (M8) handsets. From there, Android L will be next. Better study up. There will be a pop quiz on this stuff later this month.
source: @moversi via AndroidPolice, AndroidAuthority