Strava

when they show the power statistics of your ride but you don't actually have a power meter than how
realistic is it?

It's an approximation. It's only going to be close if resistance from air/road are totally benign.

If you look in their FAQ / How stuff is calculated thingy, they claim that its usually less than 5% out compared to readings from power meters.
I'd hazard a guess that they will have used the same rider(s), and bike(s) for their tests, and entered exact weights for both each time... You'd hope so anyway for a claim like that.