Explain this VREL discrepancy?

[GnawStikDog]

Finished working on a ship I bought that drifted outside the K-LEG Nav/ATC circle and since batteries were running low I figured time to RTB and refuel and restock both ships.

I set the new ship on 660-VREL towards OKLG, because batteries were running low and needed to get their faster. I set the second ship to 120-VREL to OKLG so that I'd have plenty of time to manage the first ship's navigations before the second ship potentially came within crash distance of everything around OKLG.

The slow ship, gained and overtook the first ship. I wondered if mass might be an issue, the slow ship is carrying a lot more stuff. But they are both relatively the same size. I don't know where to check the mass of each ship. And I don't understand why mass would even be calculated after VREL,I thought mass would be calculated as you burn thrusters to achieve VREL. So maybe I've got all this science wrong.

Why is 120-VREL going faster and overtaking 660-VREL?

[Ranix]

the simplest discrepancy is an assumption failure, you had 120 vrel to the wrong target (your own ship)

[Geeves]

Yep, VREL is Velocity RELative to whatever you have selected. You probably had your first ship selected when you were accelerating your second ship.

[GnawStikDog]

I double checked that. Both ships had OKLG as their target, not something else. That was my first assumption too, that I had picked the wrong relative target.

On second thought... It could still be the mistake I made. Perhaps I changed targets after setting my speed but without registering I changed targets later.