p174 and 175 list the heavy bolters on Land Speeders as having the statistics:

150m range, ROF -/-/10, 3d10+1 X. Pen 6, Clip 100, Reload 2Full, Tearing.

All three of the Land Speeders have these "gunner-operated heavy bolter" stat lines. I thought perhaps that the weapon stats had been upgraded because they're on a vehicle, but the Land Raider and Predator do not boast any enhancements (at least not as obvious as an extra d10 worth of damage).

I'm presuming this is a typo, or in fact a series of typos?

well yes, it is a little bit surprising now that you mention it. I would guess it is either a typo or copy and paste error from another part of the book.

but btw the land speeder one is weaker. 3d10+1 is an average of 17.5 compared with the standard heavy bolters 21. even taking into account tearing and the extra chance of righteous fury the 2d10 option is superior