Kelp on bedrock edition does not have any form of age system in place, this make it so all kelp plants always grow to maximum height (if they can). Making it so every piece of kelp in the oceans in 15 blocks tall, and there is no different height plants.

On java edition, when ever you plant/break a piece of kelp, it will get a random age from 0 to 25, and this determines how much more kelp can grow above that piece. this makes kelp look much better in the oceans, and balances kelp farms.

It could also be argued that this would help with performance a bit, as kelp stops growing sooner

Attached you can see an image of bedrock oceans where all kelp grows to the same height, and an image from java showing the age system on the right, and that all kelp stop growing at different heights!