iPhone 9 totally does sounds weird but the SE 2 wouldn't sound right either so might as well pick a number that's higher than 8 but lower than 11. The only thing people really care about is the fact that you can get the same processor as the iPhone 11 for almost half the price, with the Apple experience that Apple users like.

For the same price as the Pixel3a, which is $400 not on sale, this lowers the bar for customers to finally upgrade from their 7's on down. I just upgraded to a iPhone 11 and with the trade in credit of $350 from T Mobile, it whooped the Pixel's offering by a long shot money wise. With trade in credit for old SE's or at least 7's and up, this could expand Apple's A13 reach and allow them to release better services, which is what they're really going for to get past the whole "not just iPhone" myth.

It totally doesn't make sense to release a phone with lower number than main line up. It will cause a feeling that the phone is much worst/weaker.