DB Multiverse

You became much less powerful with time. Infinite energy... there is no such thing.

Our energy gets down with time, making us weaker and weaker.

You just don't realize it.

Huh?

If you don't do a maintenance check-up on your body, in ten years top, you won't even be able to walk!

Grrr!!

...

So, still not wanting my help?

What did I miss?

Yamcha is a cyborg!

That's rad!

The cyborgs in Trunks's future (U12) are less strong than the ones attacking in DBZ (U18). It's said clearly in the manga. They are also nicer in U18. The given explanation is "it's different because of the time travels".

Some fans speculated that Gero saw Trunks kill Freeza, and seeing that much power, decided to make his cyborgs stronger. It's wrong because Gero didn't know about SSJ when Goku transformed in front of him. So he didn't see the fight against Freeza.

This page gives my personal explanation. In fact, the cyborgs are weaker in the future, because they are 20 years older and are worn-out.
This explanation is DBM canon, but nothing supports it (nor contradict it) in any good DB source.

绘画：:

Gogeta Jr