Vegeta knows Freeza's power is in another league just from sensing it and knows he can further transform.
Or Freeza was just suppressing and isn't going around walking around in FP all the time. We know any power below 480K-500K or so can't break the newer model Scouters since Goku activating...
So it's just speculation.
Ginyu seemed to be able to locate Freeza's location through his Scouter when he arrived at the spaceship twice. Considering Freeza's last subordinates were already dead in the manga (Appule and Zarbon), they had to have read Freeza's BP.
Was it ever stated everybody in his army knew his PL? He seems to keep a lot of secrets from his henchmen judging by Zarbon being the only one to know he can transform.
And how do you explain Scouters not breaking every time Freeza gets near someone using a Scouter? Scouters don't seem to be...
Maybe it's a bigger one that they have at the ship, like in that filler where they read 100% 4th form Freeza and SSJ Goku. We know portable Scouters never read his power since Vegeta was slightly weaker when he broke Freeza's Scouter. That's why I think Freeza was already suppressing himself in...
Nobody ever read Freeza's power on a Scouter (the Scouter would break anyway since Vegeta broke it), he just knows his FP in 1st form is 530,000. He even referred to it has his full power in this scene, rather than saying he'd pull his punches...
It's just bad writing. Not even non-suppression users seem to walk around with their full ki all the time, but the difference seems to be that a Scouter can pick up when you're actively using ki control to suppress your strength while doing whatever Nappa and Earth Vegeta did doesn't lower your...
Freeza's seems to be straight suppression and power ups considering he openly said to Nail that he won't use full power to fight him. Also Piccolo said his Ki grew larger. And I don't think ki sensors can tell the difference between suppression and not using all of one's ki like Nappa the way...
I'm referring to this scene. Considering Freeza already did power ups in his last form which both the three in front of Freeza and Piccolo noted, this should come as no surprise. The dialogue should've said Vegeta was surprised he still had more power to show.
Vegeta acted like it was the first time he'd seen Freeza change his power level. It's as contradictory as it gets considering Freeza did a power up in front of everyone in 1st form already.
Toriyama later forgot that he already showed Freeza can adjust his power level prior to going 2nd form so that's likely. Although Goku's KHH against Nappa was a quick, uncharged one at point blank range so Vegeta might not have seen what the beam was like.
I mean fusion almost never works (even the one instance where it won a fight, Gogeta's blast missed due to the Dragon Balls), so having it actually finish the enemy isn't that bad.