Arb Waveform Generators usually use Direct Digital Synthesis to generate the output frequency, and these typically use a binary power-of-two NCO (Numerically Controlled Oscillator) to set the output frequency. The arb generator also will multiply the input reference up to a much higher frequency, using this high frequency to clock the NCO. There will be many output frequencies that can't be generated exactly with the NCO, as the necessary divide ratio isn't possible with the binary NCO. You can get extremely close, but not exact.

Simple example:

10 MHz reference, multiplied by 50 to get a 500 MHz internal clock.

16-bit NCO (They are usually wider than that, even if the Analog to Digital Converter is only 12 or 16 bits.)

This NCO can divide the 500 MHz clock by n/2^{16}, where 'n' is the frequency control value.

To get an exact 10 MHz output you would need to divide 500 MHz by 50. Unfortunately, with the 16-bit control word this would be 1310.72 / 65536, so the closest we can get is actually 1311/65536, which will give you an actual output frequency of about 10.000213 MHz.

With a wider control word you can get arbitrarily close, but often not exact.