My dd was babbling at 4 months, she would lay in her cot and start with mamamama then dadadadad then nananananan 
we lived with mil at the time. She was an early speaker, wish I had written it all down as I have actually forgotten when she did most things 
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I remember her first word that I heard though (that wasn't Mum/Dad) and that was orange, she did speak in sentences very early and lots of people remarked how advanced she was, including the hv who was actually there to talk to me about pnd(after ds1), but ended up faffing around with dd.
I didn't actually realise how advanced dd was because she was my pfb, my ds1 had a speech delay and only said words after 2, I can't remember if he babbled though, I'm pretty sure he did, but more random sounds rather than the mamam, dadadad ect.
Ds2 is only 17 months and he didn't do the mamamamam babbling either, although he said his first word before one and has quite a few words now, I'm pretty sure that by his age dd was putting words together, she also had quite clear speech.
I don't think it is just the expressive speech that counts, dd understood what I was saying far far earlier than either of my boys. She could count to 20, knew animal noises, point to parts of her body and generally had more receptive speech at a much earlier age.
Just different children/abilities really, I don't think you can predict how they will do in the future, but you notice the differences.