I think it originally said Date Uncertain, then Dec 25th has been written over it.
What I suspect happened is that part way through 1876 they remembered that the marriage had taken place, and put it in the register but without the exact date. Then someone remembered the date, and it was added later. Ancestry, in the absence of any other evidence to hand, transcribed it as 25 Dec 1876, which is certainly what it appears to be.
However, this cannot be correct, since the registration, which would have happened at the time of the ceremony, was in the December quarter of 1875. Incidentally, the GRO index has BROOKE rather than BROOKS, and I think that's what the register says too.
Useful though it is, it must be remembered that this isn't an "official" marriage register as prescribed by law, but the chapel's own register. (Note that there is no record of witnesses, and an official register would have been filled in when the marriage took place.) Many non-conformist churches relied on registrars to come and register marriages; they brought the official books with them, and only these can form the basis of marriage certificates.
Looking back towards the beginning of the register, it confirms that it belongs to Trinity Conregational Church, Dewsbury. "Rites of the Independents" on the certificate would have been written in the official register, almost certainly by the registrar, and he might have made a mistake or been unintentionally vague.