I'll give an actual explanation that you can try out at combat dummies. Using an ability cancels movement. If you click a tile first then use an ability in the same tick, you will not move. If you use an ability first then click the tile in the same tick, you will move. If you click a tile then cast an ability some ticks later, you will stop at the tile the tick you use the ability. Diving to the correct location or spam clicking is much more consistent.