If I recall correctly one was created by a non-Telegram dev as a bit of a fanproject, and eventually adopted by the company as a pseudo-official app. At some point actual Telegram devs decided to create (I think) Telegram for Desktop based on native stuff like Swift. But both applications get bugfixes and updates because it's easy enough to maintain both.

Telegram at least used to support a second non-official client.

Lately, IIRC, they have made huge chunks of their core app available as a library to simplify the creation of more clients.