Rounding errors, no doubt about it.

Note, that we don't know exact values of numbers used. They were all rounded and we were not given their accuracy. In practice that means that the last digit is somewhere between +/- 0.5 (note that decimal point refers to the digit, not the number) - so 22.4 can mean anything between 22.35 and 22.45 (more precisely 23.49999...), 0.0821 anything between 0.08205 and 0.08215 and so on. That in turn means that RT/(pV) is something between

0.08215*273.5/(0.995*22.35) = 1.01033

and

0.08205*272.5/(1.005*22.45) = 0.99097

(note: first case - cumulation of positive errors in the nominator and negative errors in denominator, second case - exactly the opposite, that guarantees maximum possible negative and positive error; this is a variant of 'crank three times' method).

We can use much better value of R & T to get smaller error, other numbers are hard to modify.