Pretty simple, in gen 3, shroomish learns spore at level 54, which kinda ruins breloom for 50s. This was changed in gen 5 to allow shroomish to learn spore at level 45. Therefore it is canon, as it was a fix made to allow breloom to have more of a chance in 50s. As breeding is not available, this should be implemented in the mmo.
tl;dr let shroomish learn spore at lv45