Yes but they were all near light-speed ships sent over a period of 90 years, and the speed variance couldn't have been that noticeable between ships because of Rackam's flight. Keep in mind that time dilation is hardly linear to speed. The time dilation is implied to be pretty high, but even if it were only, say, a factor of 2, (two years here equal one on the ship) they'd be traveling at 86% of c. 3 would be .95 c, 4 .97c, Now pretty obviously even those original ships were doing well over 90% of light speed, and the improvements had nothing to do with going faster but with accelerating faster. They didn't learn to go faster they just learned to get faster quicker. Point being though that everything that takes places in EG take place within 80 light years of Earth, maybe 85, and that includes the Bugger Homeworld. There's also the issue that if the original fleet had been much slower so the rest could reach the all the Bugger worlds it wouldn't have been a small antique fleet Ender had, because they'd have sent faster reinforcement to catch up. No matter how you cut it the Bugger empire basically had to be a cone radiating from their homeworld toward Earth, not a sphere.