On the right-hand side of the image on Ancestry, one of the icons (the one above the spanner) opens an information panel. That contains the source details, telling you this comes from a parish register held in the London Archives*. Ancestry have made an agreement with them to make these images and that gives them exclusive use, at least for a period.
So it isn't a certificate - that is a written transcript of the same information taken from the second register, which was filled in at the same time in the church and sent to the local registrar. For more recent marriages, ordering a copy from the GRO (on paper) gives you a true certificate; for older ones like this you get an image (though a rather inferior one) of their register.
FMP has no access to those images, but has two records. One they made from the GRO indexes published after the end of the quarter, merging the separate entries for the bride and groom. That has no date, just the quarter, as that's all the index tells you. The second record also has the date, and is credited to FamilySearch. This most likely comes from the transcripts made by volunteers over many years and sent in to go into the IGI. In principle it should contain the parish name too, which also is not in the GRO index. However, these FS/IGI records are very variable in completeness and reliability, as you'd expect given how they were collected.
* The London metropolitan Archives dropped Metropolitan from their name only last August, so I'm impressed that Ancestry have altered this source information so soon. Note, however, that the citation at the top of the panel still uses the old form! Perhaps they have a policy of not updating formal citations?