What I expected: hostile mobs spawning up to hostile mob cap = 79

What happened: After updates (1.4.4 and 1.4.5), in my single-player survival world, I don't encounter any hostile mobs on HARD difficulty even in dark caves or unexplored regions (entity count stays under 20 total when I enter unexplored chunks). Problem exists with new worlds, also.

Troubleshooting:
I moved my save games to another folder, then deleted the entire ".minecraft" folder. I did a fresh install from minecraft.net and made a new world as superflat "redstone ready" but hostile mobs still spawn at a very low rate and don't reach the cap (see picture)

Uninstalling, then Re-installing java 7 did not help. I also tried uninstalling java 64-bit, then installing the 32-bit version by itself, but it did not help.

This issue is unique to the 1.4.4 and 1.4.5 updates