It seems you have your plasmid in different forms. You should keep in mind that the markers you have is very likely linear DNA, and this runs in a different speed than for example, plasmid super coiled or circular.

This link might be helpful: https://bitesizebio.com/13524/how-to-identify-supercoils-nicks-and-circles-in-plasmid-preps/

thank you Rafael Cuadrat

If you really wish to know the size of your plasmid you should digest with an enzyme that cuts a single time. As Rafael points out, supercoiled plasmid has an altered mobility from circular or linear, and that is the primary form in most plasmid preps. I presume you have just run uncut plasmid on this gel?