It might be based on the handwriting standards used in your country. Where I live we were taught at school to draw a horizontal bar on 7 and avoid the serif on 1:

https://is.mediadelivery.fi/img/468/a93c32e08dae4768869a4bda...

No chance of confusion. This seems to have prompted some to add the serif to their 1 for stylistic reasons or whatever, since it's still distinguishable from 7 with a bar.

But then again people following older or newer conventions drop the bar from their 7:

https://is.mediadelivery.fi/img/468/46827e3320294f89b12a9338...

This makes a singular 1 with sloppily drawn serif hard to distinguish from a 7 without horizontal bar unless you can also see how the same person draws the other digit in their style.