Cycling on a flat road requires one to move a volume of air out of the way. The power required to go faster is not linear meaning it takes a lot more power to go just a little faster. That is why improving time trial times is so difficult.
For example... Using
http://www.noping.net/english/ it takes a 175 pound rider approximately 190 watts (in the drops) to maintain 20 mph. To go 25 mph, requires 330 watts which reduces the time to 25 minutes. Increasing the speed 25% required an increase in power of 74%. And year over year improvements in human power performance in general are 5 to 20% and sometimes we stay the same or decrease. So, when one "hammers" the power does go up but not enough to make a significant difference in time or speed. However, knowing this, you may be improving quite a bit but the measurement method is not accurate enough to gage improvement.
Hopefully, I have my math right or C'Fool will correct me.
