Do brown mushrooms grow faster than red ones?

Early experiments in mushroom farming have resulted in a noticeably higher yield of brown mushrooms than red ones. Is this an anomaly or do brown mushrooms genuinely 'grow' faster?

To quantify, 5 times as many brown mushrooms have spawned in my brown mushroom farm, which is identical in every way to my red mushroom farm. The brown mushrooms were planted only a few mintues after the red ones.

No. They have exactly the same spawning logic. Each mushroom has independent spawning process. This assertion can be verified with decompiled code for Beta 1.6.5 using MCP 3.3 where the mushroomBrown and mushroomRed are initialized in Block.java as instances of BlockMushroom class with almost the same parameters and the mushroom spawning process BlockMushroom::updateTick() is not affected by the difference in instantiation. The specific code is proprietary which I don't have rights to redistribute, but you can still obtain it with MCP and your own copy of Minecraft™.

Based upon your description of your test in the comments, perhaps it's related to elevation. Whenever I find mushrooms in caves they tend to be quite deep, down near the lava layers.