Had it happen to me. The reason is new phone, or sim card contacts arent saved. Everyone attaches a name or picture to a number anymore that comes up on their caller id, and if its not set like that they dont remember. I recognize numbers by carrier. The 1st three digits usually signify certain cell carriers so its easy to remember(for me) in fact if a contact isnt saved and tagged by name.
Or if your in that "realm" your number is not adhered to their phone for reasons of privacy or to keep you secret(if your in something FWB or non committal). If someone sees their phone with 20 text messages not erased and its ###-###-#### instead of "Rando" or "Cody" etc its easier to keep things at rest. Same thing with Call log, ###'s versus a Name being shown and a call time of so many minutes/hours. No proof of who it was. Theres alot of jealously and noisyness going around and people like to stick their nose in sh1t all the time, infact I would lie myself if someone seen my phone and started asking questions just to keep them out of my business.
Ive always said this is so and so from such and such if they forget, and they could be playing around as well even if they had your number to keep things playful and flirty from where you last left off with them.