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Quartertones out of tune?

either there's a bug or I have a misunderstanding how the quartertone tuning of operators works. I'd expect two quartertones to match one semitone, but that's not the case. Going up from zero, +2 quartertones is more than one semitone. Coming down from +24, a settting of +22 is less than a semitone down from +24. What's going on? This is v6.0d with the Init Patch.
Best, Boris
This is normal, although a bit misleading. The frequency gap between multiplier=x and multiplier=x+1 is divided linearly into 24 steps. That doesn't match exact quartertones/semitones on the musical scale because it's not exponential.
However in FM that's not a big deal, as the final ratio between operators is the only thing that matters. (multiplier=1 and quartertones=12 is 1.5 ratio. multiplier=2 and quartertones=6 is a 2.25 ratio for example.).
Simple ratios will sound the most musical, then as you go with more and more decimals it tends to sound inharmonic.