This question has troubled me too.
I think the Bolo is simply the continental version of the Avon, developed to fish the deep, steady moving rivers typically found on the continent.
Similar to our Avon which can come in the form of a 'shoulders up' float which is typically used in a wider variety of circumstances such as fast, shallow swims where boils and swirls might be present and where you'd want a float with a body shape which subscribes to slowing it down rather than running it through at pace, or indeed the rugby ball or 'bolo' shape of body for steadier swims where just a low bulk shot is desirable.
I suppose you could say that a rugby ball upper body shape is a bolo and anything with a tapering body leading to shoulders is an Avon but that's simply not true.
Avons are a traditional part of our fishing heritage and I believe bolos are simply the continental equivalent which were imported along with the namesake method of fishing which suits so many of the big, slow moving rivers there are on the continent and we've just adapted it to suit some of our situations.
I don't think there is a significant difference, both bolos and Avons can carry the same rugby ball upper body and, looking at the various different types of both, tip widths seem to be incidental rather than deliberate but both will do a similar job except a bolo or more rugby ball shape might not cope with all the situations of something with shoulders, hence we see both versions labelled as Avons.
Go a bit further...
The Thames Roach Float.
Another variation on a theme, start with an Avon and slim down the body shape a little in consideration of the species.
But again that principle considers only the species and not necessarily the swim and the presentation that's required.
I think a lot of these labels come from individual circumstances, we love specifically labelled items of tackle especially if we think they will catch us more fish but they're all just variations on a theme.
When it comes to the nitty gritty I would wager that the stem material is of more importance than subtle variations in bodies and tips although I've only ever seen wire stemmed bolo floats.
So maybe that's the answer...
A bolo float will have a wire stem whereas an Avon might be wire, cane, quill or reed and the Avon comes in two different definable body shapes to cope with a wider variety of situations whereas the bolo is a continental equivalent specific 'Avon' for just one?