The 40 isn't the height, but rather a ratio to the width.
I.e. the 225/40 has a 90mm tall sidewall or 3.54"
The 235/40 has a 94mm side wall or 3.7"
Good tool to help you see the difference in various tire sizes
As you can see the circumference has changed. Now the 225s are going 837revs/mile to the 235s 827revs/mile. This difference will have to be sorted out via the center diff and depending on the size/amount of the difference this would not be good for it, i.e. overheat and cook the diff. The difference in your case is very small, but you should check with manual IIRC it states no more than a 1 or 2mm difference in which case you would be over. Is this a case of Subaru being over cautious and just trying to CYA.......
Food for thought; some tuners suggest using the same size tire, but using a wider rim on the front. This is what your thread title says, but you ask about tire in your post. A 255/35 would be closer to the 225/40 than the 235/40.
I'm no expert so take what I say with a grain of salt.