I think there may be confusion here between (V) volts and (Ah) Amp Hours.
I run 12v 14Ah on my 300 and 250, never get an issue. If this is the case for you then you'll be fine.
I've NEVER heard of a 14V battery for a bike or ATV, 6V yes but 14....?
Edit: Trx250a' were fitted new (in the UK) with 12v 10Ah and 12v 12Ah. The differences or year of machine the different batteries were fitted to I don't know.
simple answer here too many volts can be BAD, you can start to short things out, or possible even a fire, as wires are only meant to handle a certain amount of power?
I have never seen a 14 volt battery that looked like a 12 volt replacement
so I too think MAYBE your just reading it wrong, or maybe your getting a volt reading while its running and you have some other problem that is making it charge to a higher reading??