Relative teleport commands appear to be measured as relative to the target but applied relative to the context, resulting in unexpected behavior. For example, this function:

tp @s ~5 ~ ~
tp @s ~ ~ ~

Expected behavior: player is not moved at all
Actual behavior: player is moved 5 blocks in the negative X direction

It appears as though the player moves +5X, then the second command tries to put them back to the context position, which is a relative move of -5X from their current position, but it applies that -5X to the context position instead of the player's position, resulting in the player moving from +5X to -5X.

For another example:

tp @s ~ ~ ~5
tp @s ~ ~ ~5

Expected behavior: player is moved 5 blocks in the positive Z direction
Actual behavior: player is not moved at all

Again, it appears that the player moves +5Z, then the second command tries to put them there again, which is a relative move of 0Z from the new position, but it applies that 0Z to the context position, putting the player back where they started.