There's a definite difference between one's flight speed and combat speed. This can be seen several times in the series, such as Goku running across the 1,000,000km Snake Road taking roughly a day whereas a far weaker Piccolo's Ki blast got to the moon in seconds, or Gohan thinking that testing out his speed against Kinto'Un, which has only been shown to fly at supersonic speeds, was in any way a noticeable benchmark for any bukujutsu user in Z (bar Videl). Even if Ginyu has greater combat speed, there would be nothing inconsistent about Butta having greater travel speed when the correlation between them is consistently inconsistent, especially when it could be part of Butta's mutation (assuming all the members of the Ginyu Tokusentai are mutants).
sei'taer said:
Butta's claim was always a pretty easy bluff.
Ginyu was the one who first made the claim, so there would be no reason for it to be a bluff.