10 Inch Tonearms

As tablejockey said, 10" is a compromise —in a good way—giving some of the advantages of a longer arm with those of the 9". Legend has it that 9" is faster due to less mass, while 12" does the better tracking thang.

There are two other variables depending from TT construction.
There are TT's which allow only 9'' tonearms ( Kuzma Stabi
Refernce) as well TT 's which don't. Technics SP mk 2 for
 example does not allow 9'' .

At the extremes (commercially available, that I'm aware of more) is the ViV Rigid Float arm at 7" and the Kuzma 4P at 14". What does that tell you? Any length will work and they all have their pluses and minuses.

Viv also make 9” and longer versions of their tonearm, all the way to 14”. Viv is an underhung tonearm, entirely different from Kuzma or nearly all others, that overhang the spindle. Underhung tonearms generate much more tracking angle error than overhung tonearms, which error is directly proportional to length. I think Nandric nailed it; some TTs simply do not fit 9” tonearms, so there is a market for 10 and 10.5 tonearms for those TTs. And I do NOT agree that 12” tonearms are inherently superior to 9” tonearms “in every way”. Thus some choose 10 or 10.5 as a compromise .
EDIT.  I misspoke. Of course TAE for overhung tonearms is also proportional to length, which accounts for the existence of 12" tonearms.  What I was thinking is that the very large amounts of TAE that result if you use a 7-inch underhung tonearm like the Viv are drastically reduced if you use the 14-inch version. If you're obsessed with minimizing TAE across the surface of an LP, then you might opt for a 12-inch overhung tonearm (with the attendant increase in effective mass and greater propensity for resonance), or a linear tracker.