I've noticed this in other games, too. Interestingly, in one of the recent comparison videos on youtube channel Retrocore about Pac Man, there were some systems where it moved faster horizontally, but one that seemed to move faster vertically.
Weird Eltigro! Also, as per a YT vid I saw, speech isn't all there using AtariVox. Hear a super quick word or two? and sound effect once in a while... can barely make anything out. Sent my AtariVox out several years ago for an update... wonder if there's been another update since?