It's a combination of several things.

--A J-frame + speedstrip(s) is more compact than a high-cap concealable semiauto, so people tend to pack on less extra ammo (since they're selecting it for peace of mind, comfort, and concealability).

--It's easier to carry two 10-round 9mm magazines than it is to carry four .38 Spl speedstrips or speedloaders. Magazines in most states hold way more than a J-frame (and in the rest, they just hold a few more).

--The default number of spare magazines for a lot of people is two, because they're spare magazines, not necessarily extra ammo. Oftentimes the spare magazine is there in case of a lost or damaged mag.

--People tend to lie about what they carry on a daily basis.