An Intel Core Ultra 7 155U and a Core Ultra 7 155H, are very different classes of CPUs!
If you're comparing laptops, you'll see both listed, and laptops with the U variant will be significantly cheaper, because you get half the max TDP, 4 fewer cores, 8 fewer threads, and a worse GPU.
This isn't to say the 155U is a bad chip, it's just a low-power optimized chip, while the 155H is a high-performance chip, and the difference between their performance characteristics is a lot larger than you'd expect when looking at the model numbers. Heck, if you didn't know better, you might text your tech-savvy friend "hey is a 155 good?", and looking that up would bring up the powerful H version.