24mm not always equal to 24mm

In case of Canon lens math doesn't seem to work. I mean 24mm in 24-70 lens is not the same as 24mm in 24-105 lens. At least this is what I've discovered recently testing my friend's 24-105 lens. He bought used lens and asked me to compare with my 24-70 or check if that 24-105 is sharp enough. So I set up my tripod with 1D MkIII on it and first attached my beloved 24-70L/f2.8 lens to photograph scene consisting of bottle of Baileys and few russian dolls. After taking a series of pictures (24, 35, 50 and 70mm at apertures like 2.8, 4.0, 5.6, 8 and 11) I've changed lens to 24-105L/f4 IS and taken another series of pictures. After downloading them to computer I was surprised that at 24mm they had quite different frame coverage. Then I read that angle of view has a difference at about 10 degrees between these two lens at 24mm. I still don't understand this phenomenon, but have only pictures to proof it...