i was thinking the same thing WD, water authorities use all sorts of chemicals in your water, they work on the basis that water is used for cooking, washing and consumption mostly, for all these things the water is only used for a relativley short period of time and then drained away, except for thos of us nutty enough to keep big glass boxes of water in our houses, most people only use water short term.
this means the primary concern for water companies is often how the water will behave for a couple of hours after being drawn out of the tap, they use things like pH stabilisers which work for the time we need to use them but their effects will wear off if left. we've come across it before when peoples water readings out of the tap for pH are fairly neutral but after a few hrs start to drop down.
I don't know enough about the science behind water processing to speculate much further, but i can't help wondering if they are adding something which supresses the ammonia for a short period of time. This might be one for discussion in the science forum.....
However that being said, what concerns me more is that your filter is unable to process this little extra blip of ammonia after a water change, there are plenty of people who have tap water with an ammonia content of 0.25ppm, they would see a small blip of ammonia just after a water change but the filter would soon process this and if you took a reading a couple of hours later it would be at 0. The fact that your filter is not doing this is more of a concern than the ammonia rising, we should be careful not to get distracted.
Remind me what size tank this is, what make and model of filter you have and what fish and how many you have, run through your routine for maintenance, filter cleaning etc etc and we'll see if we can spot the problem.