What's behind my plasterboard?
There are a few possibilities; without knowing more about the layout of the house makes it harder to say for sure.

The internal wall in the garage might be constructed in normal blocks for fire performance reasons. (Without looking, I'm not sure about the fire performance of thermal/lighweight blocks). Also, normal blocks might have been used for structural reasons - for example if the wall is load-bearing (normal blocks generally have a higher compressive strength than thermal/lightweight blocks).

Likewise with the bathroom and en suite - there might be a difference for structural reasons.

Or there might be no firm reason for the difference at all, other than the builders using what they had available - it's not unusual for builders to use what they have as long as it meets any specific requirements for the particular element of the structure.