Water heater question

Thanks in advance for your input.
I have just replaced my gas water heater.  But both my old and new one has an interesting problem: the water coming out of the water heater has different temperature on different days.
I have not changed the setting on the water heaters.  the water comes out of the same location in teh house.
It happens 10% of the time.  What I mean is: on 10% of the days, the water coming out would be much cooler than on other days.
I cannot think of any reason other than that the gas coming in is weaker?  Does not seem right.