RealTime represents seconds, as opposed to ScoreTime, which is in abstract units. Everything eventually is transformed into RealTime to be performed.

This type has switched from floating point to decimal and back again. The problem is that floating point is not exact, but there are a few operations that require events that have the same ScoreTime to be grouped with each other once they reach RealTime. For instance, controls are clipped to the note boundaries, and a note is required to have a pitch at exactly its starting time. While the event that produces the pitch signal may have the same ScoreTime as the note it belongs to, if imprecision has caused it to drift a little by the time it gets to performance, the note may wind up with no initial pitch, or pick up the pitch of the next note as a pitch bend.

An example of how imprecision can accumulate is a block call with pitch set in the caller. If the sub-block has a note at 0 this should line up with the start of the block call in the super-block and hence with a pitch at the same time. But the sub-block has its own warp which is a composition of the its tempo and the super-block's tempo. In theory the sub-block's warp should be shifted so its 0 starts at the calling point in the super-block, but in practice this is a number of floating point operations (addition, linear interpolation, ...) and the value may very well be slightly different.

Unfortunately switching RealTime to a lower-precision decimal type has the same problem because it introduces even more imprecision due to the ScoreTime -> RealTime -> ScoreTime conversion (this happens during warp composition, for instance, since shift and stretch are in ScoreTime). And I think it's ultimately not quite right because rounding will still produce incorrect results if the imprecise value falls at a rounding boundary.

Eventually, for MIDI at least, everything is rounded down to milliseconds so hopefully any imprecision can be accounted for by the operations that care about it and eventually be removed from the final result.