I wouldn't worry about it. There are only two options: you can drain, clean and refill; or you can chlorinate till it's gone. Unless you are asking the question, "Should I drain the pool?", the question, "Exactly how much ammonia is there?" isn't really relevant.
A variation between FC=13 and FC=14 is certainly within the error range of that test . . . so it may not be an actual difference. But, depending on circulation patterns in your pool, it may not be an error. In that case, the 'best place' to get a sample, depends on which part of the pool you want to measure!
Are you thinking about draining and refilling? (Keep in mind, this is not really an option with a vinyl pool.) If not, just chlorinate till it's gone.
Just chl
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