Absolutely normal.
The key thing is to understand there is a difference between the register (an original document, signed by those involved) and a certificate - which is a COPY of the register entry, written out later (or even at the time), but does not have original signatures on it (except that of the person issuing it, usually).
Certificates can only show the signatures of the parties involved in a wedding, or the informant on a birth/death if they are photocopied from the original registers, and GRO don't have those. They only have information copied and sent in to them each quarter by the registrars.
If you order a certificate from the local office that holds the register, they might create the certificate by photocopying it, but they don't have to - they may just copy it out for you by hand, or type and print it, themselves.
I issued hundreds of marriage certificates as a registrar at weddings, NONE of them were ever signed by the bride/groom or witnesses. All that is required is that the registrar writes the signatures on the certificate in the same form as the original ( e.g. A Jones or Anthony Jones or AJ, depending on how the person themselves signed), but they don't try to copy them exactly.
If the marriage took place in church (which involves two registers being completed), you may find an image of the church marriage register on-line on Ancestry or FindMyPast, depending on when/where it took place.
But - there are exceptions - i checked my own marriage certificate recently and found that I did sign that myself, as did my wife and both witnesses, but that is because the vicar didn't follow the rules correctly. There was also a brief period, I think in the 1980s, where at least some offices submitted their quarterly returns to GRO by photocopying the original register pages (inc births & deaths), so you may see a signature on a certificate from one of those, even when ordered from GRO.
