I have a MacBook Pro Mid-2009 running Yosemite 10.10.3, and I installed Macs Fan Control which is similar to smcFanControl. Basically, whenever I put my Mac to sleep the fans stop spinning (I have them set at 6200rpm) and the Mac is asleep but less than a minute into sleep mode the fans start spinning again (I can tell since they start at 6200rpm) but the screen is still off. Is my Mac really going to sleep or does the fact that the fans are spinning mean that it's not asleep. By the way I have wake for network access turned off.
Fan function is regulated by temperature sensors.
As long the temperature is to high the fan keeps working regardless if you are in sleep mode or not. Since you turned off the heat source that action is usually very short.
Somewhat like you car fan, that continues to spin even if you turn off the engine if the temperature is to high.