I guess this only noticeable in jars and not tubes...
Noticed this a few times that the amount of product seems to reduce (and just due to use by me) and especially towards the end of a jar - it really picks up speed..
I opened a jar of moisturiser 3 weeks ago (big pot, supposedly good stuff - Elemis) and I only use it in the morning, a little bit, I don't have a massive face. Already half way through...
And there's no one in my house into face care to nick some.