Stupid question: actually what makes it an outdoor speaker vs indoor? shouldn't all speakers targeted at a flat on axis frequency response?
Not a stupid question, but I note nobody answered.
When you listen to a speaker, there is the direct sound that reaches your ears and the indirect sound from reflections. Outdoors is mostly direct sound as there aren't walls, or they're too far away or a hedge etc to make indirect sound usable in sim as with direct sound.
Indoors, the wall/ceiling/floor reflections will reach your ear, albeit with a subtle delay. Floyd Toole's work suggests that the energy expended by a speaker in producing direct and indirect (reflected) sound should broadly re-sum at the ears, such that SPL levels are broadly smooth and even at the listener position - as much like the input signal as possible. This is made easier when a speaker radiates all frequencies as evenly as possible over a similar arc in what's called constant directivity, although controlled directivity is probably a better term, since I don't think there is a speaker exhibiting true constant directivity, but some come close.
Since a room contains the expended energy and reflects much of it (frequency dependent of course, and dependent on absorptive vs reflective furnishings in a room), then what is heard is a summed response of direct and indirect sound.
Outside is a different affair. Since SPL drops off over distance, and since all frequencies, but especially bass won't be reinforced in the outdoor scenario, then having some surplus output helps maintain the SPL travelling towards the listener. This is especially true if the speaker is intended to be used away from walls on a tripod mount or similar.
This is why I object to measuring an outdoor speaker by indoor sim models. Their response was designed for outdoor use and not to make use of indoor reflections. Although, if they have been reviewed because 'objective' audio-buffs want to use a speaker indoors when it was designed to have a response for outdoors, then what can we do?