That’s true. The only thing I can think of for getting an hourglass shape would be potentially applying uneven pressure during the cuts. The track saw track should give you a dead straight line, but only if you aren’t applying any lateral force during the entire cut.

I found a lot of useful stuff from this video from Foureyes, particularly around the 6 minute mark for the in/out method on the jointer. There’s also a similar up/down method if you have a table saw.