In the first example, you are dividing a bunch of 1s by the sample rate. In the second, you are dividing some small numbers and some large numbers by the sample rate.
In FP, the larger the difference between the magnitude of the numerator and denominator, the larger the resulting FP error.
Try it out:
samprate = single(25/3);
f = @(N) abs((3*N)/25-N/samprate);