The idea that Freeza's men can raise their battle power but not lower it gets thrown around a lot, but I'm not sure about it. The people with scouters always make it seem like a battle power
will not change at all. Take this quote, for instance:
Chapter: 214 (DBZ 20), P4.3-5
Nappa: “981... 1,220... 1,083... Idiots! Do you really plan on defying us with that level of battle power...?!"
Vegeta: “These people change their battle power in response to the fight. Those figures can't be relied upon any more.”
It's not, "We can't rely on this reading because their battle powers might increase when they start fighting, just like ours." It's, "We can't rely on these numbers because these guys can change their battle power at will.*"
*"Unlike us"?
Another one is:
Chapter: 222 (DBZ 28), P4.1
Vegeta: “Someone with a battle ability of about 5,000...!!”
Nappa: “5,000?! That’s impossible! It has to be a mistake.”
Vegeta: “It’s truly an unthinkable figure for the old Kakarot. What’s more, the people here are able to alter their figures…So 5,000 could just be his minimum.”
"People here are able to alter their figures, so this might just be his minimum." Doesn't that make it seem like people not from there, i.e. Nappa and he, aren't able to alter their figures from a base minimum? All of the dialogue points towards their battle powers being static, if you ask me.
So what does that mean?
When Nappa & Vegeta came to Earth, they were shown to be able to increase the battle power they were using at a given time
Technically, since no scouter was pointed at them, we don't know that their "battle power" increased at all. Which raises a question that sometimes causes some controversy: are battle power and ki truly the same thing?
I think it depends on how you look at it. I definitely think battle power is a
measurement of ki, but that doesn't necessarily mean that scouters and people measure it in the same way. If we just take everything at face value for a moment and try to think objectively, these are the facts we have:
- Nappa and Vegeta make it clear that battle powers changing is uncommon, implying that theirs don't.
- Nappa "powers up", and the Earthlings become terrified by his power, hinting that they couldn't tell before just how powerful he was.
- Piccolo only guessed that Vegeta was the stronger of the two because he was bossier, indicating that they couldn't tell from his ki alone.
- When Vegeta kills Nappa, Goku admits that he was stronger than he first thought.
If I think about it completely objectively, the only explanation that fits is that a scouter can't tell the difference between Vegeta standing there and Vegeta blowing Nappa up, but someone like Goku can
feel a difference. A lot of people don't seem to like this idea because it implies scouters may have had some sort of advantage over ki sensing--that is, that it can tell a person's "true strength" before they start fighting, whereas a person who can sense ki can't. I
suppose that's true in a way, but only when pointing your scouter at someone who can't change their battle power. Plus, if we consider that Goku and Piccolo couldn't even find Raditz by sensing his ki just a year earlier, it may simply be that their ki sensing simply wasn't perfect back when Vegeta and Nappa arrived, and may have improved as time went on.
There are a few other examples that might lend themselves to the above idea. Videl doesn't know what ki is, so you wouldn't think she could change her battle power at will, yet her ki only becomes noticeable to Gohan once she starts fighting. I used to think this meant that her battle power did increase, like others have suggested Vegeta and Nappa's might have, but taking what I said above into account, it might be that her ki just became more noticeable to ki sensing. If a scouter had been pointed at her at that moment, maybe it wouldn't have registered a change in her numerical battle power at all.