I just made a discovery, 226 .40/.357 Mags fit and function in a 229. The 229 Mags will not fit in the 226. Also the 226 9mmm Mags go into a 229. Both have a little hang over on bottom. They will function in 229 9mmm also.
So what gives? Why?

Because the P229 (.40/.357) mag has always been wider at the bottom half, ever since the P229 came out. The P226 mag has always been uniform from top to bottom (i.e., no wider step like the P229 mag).

When I bought my Beretta 92 during the infamous “Assault Weapons Ban”, it came with 2-10 rd magazines. A friend of mine in law enforcement told me to get two 10-rd 40S&W magazines. Seems they worked great in the 9mm, and held 14 rds. Just tried my P229 and P320 magazines in 40S&W, and they easily hold the 9mm cartridges.I definitely would not try this for CCW, but will try using the 40 S&W mags next time I go to the range with the 9mm, just to satisfy my curiosity. Not sure what would happen in a state that has a magazine limit.
Just tried my 40S&W mags with 9mm: P229(12 rd) will hold 14 9mm, but had problems removing the cartridges, probably due to slight differences in the followers. The P320SC(10rd) held 12-9mm, just like the 9mm magazine would. The 320Compact(13 rd) held 15 rds of 9mm, like the 9mm version, and seemed to feed OK removing the cartridges(as did the SC). The 320 FS(14 rds 40S&W) would only hold 16 rds of 9mm vs. 17 rds for the 9mm version, but had no problems in feeding.
Inquiring minds want to know!

Yep. Per 12131, wider P229 mags are too big to fit in the P226 magwell.

After I read about this in a SF post 10 years ago, I ran all of my P226 .40 mags through my P229 .40. No malfunctions.

Lots of odd things work because...they just work. One example is the P239 .40. It will accept and function reliably with a factory OEM 9mm barrel (not a conversion barrel). Not only that, with the 9mm barrel both .40 and 9mm mags can be used reliably.

In addition, the discontinued Sig OEM "Plus 1" kits which give one extra round in the mag, will function with both the P239 .40 and 9mm mag bodies. Even though they were marketed for .40 cal only.

The longer you've been around guns, the more weird shit you'll probably discover.

Thank you, I am so glad my 74 year old mind is not playing tricks on me!