I can confirm the bug (if it's actually a bug; it might be intended as well, but in previous versions of Minecraft, every villager was transformed). The problem does not depend on the operating system and also happens on Creative. I have Minecraft 1.7.2 as well, and I've tested this issue on Hard difficulty, too. I spawned 50 adult villagers and a normal zombie with spawn eggs and counted the zombie villagers after all villagers were dead. 24 villagers transformed into zombie villagers. 26 simply died.
I did the same test on Normal difficulty. There I counted 21 zombie villagers in the end; 29 villagers were killed.
On Easy, no villager was transformed.
Someone on the Minecraft Wiki has confirmed this issue as well. He/she didn't get the same percentage as I did (out of 500 villagers, only 110 were killed, 380 were transformed, 10 were able to flee), but got the same result as me: not all villagers become zombie villagers after they have been killed by a zombie on Hard difficulty.
Steps to reproduce (it's best if you do this in an enclosed area so the villagers can't flee):
1. Set the game to Hard difficulty.
2. Find or spawn some villagers (the bigger the number is, the better you will be able to reproduce this).
3. Spawn zombies near the villagers or wait for a zombie siege (it doesn't matter how many zombies attack the villagers).
4. Let the zombie(s) kill the villagers.
5. You'll see that some villagers will simply die instead of turning into zombie villagers.