Thanks all, really appreciate it

I admit I used to go back and forth on this one - but at least I have two signatures to compare, so can't complain too much! I can't seem to quote at the moment so I'll try to summarise and respond to some interesting points others have made:
-If indeed I have the right Ellen, she was born in 1863, so would have been around 23 when she made the first signature, and about 36 in 1899. Which could explain the handwriting variations, e.g. the difference in 'y'. Potentially.
-Thinking about the context in which the two signatures were made: I don't know if I'm reading too much into it (I probably am), but the first signature seems almost rushed. I haven't included the rest of the image, but her handwriting is noticeably more askew and thicker than the Registrar's, as if she were writing it in a hurry and at an angle, just wanting to get it over with. I can't imagine an unwed domestic servant registering her illegitimate child's birth was a pleasant thing to do in the 1880s.
-Good point re. the spelling of the name Rebecca. I've looked into Ellen's family and there are a lot of Rebeccas in her maternal line, so maybe she decided to adopt it to honour a relation? Or, like me, she felt left out and just wanted a middle name

-A bit of background on the Ellen I believe to be my g-g-grandmother's mother: she was a domestic servant on the 1881 census living with her parents, but had moved to Dorset by 1891, still as a domestic servant. She had an illegitimate daughter in the late 1890s before she married, who later took her stepfather's surname. I have no DNA matches with this family, but the daughter of Ellen's daughter took a DNA test a while back, and my sibling shares 40cM with her. If we are related via Ellen then she would be a 1/2 1C3R, so definitely possible on the DNA Painter tool.
Personally, I'm of the opinion that it's the same person - as matthewj64 said, the distance between the 'E' and the remaining letters in 'Ellen' is present in both signatures. I can also see similarities between other letters (e.g. the number of loops in the capital 'C', which is made less obvious in the first signature due to the ink of the pen, and the spacing and shape of the lowercase 'n'). But it's always a good idea to ask others for their opinions! A number of people in my own family are unsure if it's the same hand.