Canon zooms - why so much overlap around the same price?

[Kermit power]

I've just been having an idle browse around on camerapricebuster.co.uk, and I'm a little puzzled as to why Canon have 3 zooms which seem to overlap quite so much? Obviously the 100-400 does have a wider range, but even so, three zooms all for a similar price, with similar maximum apertures... Why? How do they differ?

The lenses in question are these ones...

Canon EF 70-300mm f4-5.6L IS USM - £1,029
Canon EF 70-300mm f4.5-5.6 DO IS USM - £1,049
Canon EF 100-400mm f4.5-5.6L IS USM - £1,139

[JDRoest]

Kermit power said:

I've just been having an idle browse around on camerapricebuster.co.uk, and I'm a little puzzled as to why Canon have 3 zooms which seem to overlap quite so much? Obviously the 100-400 does have a wider range, but even so, three zooms all for a similar price, with similar maximum apertures... Why? How do they differ?

The lenses in question are these ones...

Canon EF 70-300mm f4-5.6L IS USM - £1,029
Canon EF 70-300mm f4.5-5.6 DO IS USM - £1,049
Canon EF 100-400mm f4.5-5.6L IS USM - £1,139

Diffractive Optics on the DO lens. Canon trying something different. Personally, I'd take the 100-400 over the 70-300 as well but other people might want 70mm.