Data Tags Interfere

when using data tags,
i found out they dont work together
for example;
having boots with a negative speed value (-90%)
and holding a blade with +10% speed value

will end up givign you +10 in speed, the other effect gets ignored.
could they get combined, and thus, added up to each other?

(attached is a testworld for 14w11b that i used to test it in)