Well, of course it does. There is no way Word can tell whether 'ar' or 'in' is an abbreviation. You would reduce the risk of false matches if your abbreviations followed the convention of incorporating periods in them (e.g. 'a.r.' or 'in.') and included the period(s) as part of the Find text, but even that won't stop 'in.' for example being matched here:
I found a bin to put it in.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]