API test, while being the standard and most reliable, is kind of infamous for this. The light being used to read results can have a significant impact on the color your eye sees.
Also something else to consider is if you are dosing fert it is possible you are adding ammonium (non-toxic form) but however will still read on the API as ammonia.
That being said it is possible that if you're overstocked the bacteria which converts ammonia to nitrite is not able to totally keep up with the load and hence always have a slight ammonia reading.
I've experienced very large ammonia spikes before with not a single rise in nitrites which has led me to believe that ammonia ?? Nitrite bacteria has less capacity than the Nitrite ?? Nitrate bacteria. Though that's just my own empirical findings.
All that being said if you're confident that it's cycled don't worry about the possible .25reading"
I my self obsessed over this one a few years back units I came across,many, many, many others with this same concern....
It's 2016 already....fine I still don't have a hovercraft ....but how in the hell is it that we don't have one very small probe that one simply place in the tank; it collects all water parameter and then wirelessly sends this to you as notification.