Because their info (as anyone else's who's not a military intelligence agency) is incomplete.
Basically, j7 units were there for decades and they had many photos of them using j7. There were ne photos of other plane types reaching the unit. That's why they labeled them as j7 units.
For j10a, there used to be many more units. Like 9 or so. As photos of j10a planes coming to a unit, scramble would label it as j10a unit. But then as photos of even newer types of planes reached the same unit, scramble would switch that unit to new type. Thus lowering the number of units.
So you may ask why havent even older units gotten second hand j10a. They have. But photos of such occasions are very rare.
Firstly, official photos of new types in a unit unit have in the past been of new planes. As new deliveries are exciting to show. Secondly, amateur photographers arent as interested in tracking second hand planes. So there are fewer such images around. Very few people care about taking pics of j7 and even j10a , which are now 20 years old.
Thirdly, plaaf has become more secretive in last 5+ years. So fewer official pics are released. And due to legal risk, fewer amateur photographers are trying their luck spotting and photographing planes.
Finally, plaaf has started building shelters for their planes pretty much everywhere. So while old satellite imagery sometimes showed rows of planes on an apron, last several years its just rows of shelters, making it harder to know which exact plane type is underneath.
Basically, info scramble has is incomplete and often outdated. Having saind that, it's still one of the best sources out there as most other sources are worse.
And yes, all this means it's quite possible not even 5 brigades are actually operating j7 anymore. But that scramble simply relies on out of date info when it comes to such old planes.