Collo,
I'm not too sure but it would have to do with the quality of the embies and the place it implants or whether it implants. I always had nice thick linings and went double digits, although that was on single transfers. I spoke to a girl who went thru a different IVF clinic to me and she was told not to lift anything in the 2ww not even shopping bags she got pregnant first transfer. I dunno it might be a numbers game for some, I like to think it may have a bit of positive thinking behind it too.
Good luck I hope you have a embie growing inside of you soon
Bec