Leeboy... it's not sloppiness. It just reality creeping in. What you are hearing is samples being stretched up or down to cover more than one note. This makes any periodic aspect of the sound (vibrato, tonging, attacks, decays, etc.) either speed up or slow down as the sample is transposed, and any timbral characteristic, and formants get brighter or duller as the sample is transposed, again.
Reality is that at today's cost for ROM, it would be too expensive to have enough ROM so that this NEVER happens. Something on the order of twenty to one hundred times as much ROM would be needed to do this. Look at the size of GIGA samples that do this... each one is close to GB's in size. This is no problem for GIGA Sampler, as it streams off the hard drive and doesn't actually use much RAM, but in a non computer keyboard, well, the top limit for current ROM is maybe 200-300MB or so. For the entire soundset.
So you have to be realistic, I'm afraid.
As for the Kurzweil... sorry, I've got one of those. PLENTY of abrupt sample boundaries in that (I have the K2500S). They, however, don't suffer from the vibrato thing as much as many modern keyboards, because the basic ROM sounds were sampled before they HAD enough room for vibratoed samples (they take up MUCH more ROM), and tend to use LFO's for most of the vibrato, along with some clever programming, like modulating the LFO speed from the wheel amount or velocity.
In fact, I wrote an article here some time ago showing how these sample boundaries can be used for great effect to get MORE timbral variety out of the instrument.
http://www.synthzone.com/ubbs/Forum37/HTML/017002.html Enjoy.