I wouldn't know in that case.
As far as I know, there's no reason. Itch charges a lower percentage than Steam. If you wanted to pass that difference on to the end user, then the prices on Itch should be lower than on Steam.
The best thing would be to ask those developers directly, and if they reply and you share it here, I'd appreciate it, as I'm intrigued (it's the first time I've heard of setting different discount rates).