Well, if you measured the width of the bottom bracket as requested, you would have had an even bigger answer.
Here's the full story.
Shimano Hollowtech II cranks come in one size and by size I mean the length of the crank's axle. You'll notice on the other hand that square taper and ISIS and Octalink BBs come in many sizes (lengths) to satisfy the various styles of frame out there.
Now, if you only make your crank in one size, then you have to do something to make it fit many frames. So you make it long and supply spacers to take up excess space.
Since there are two frame standards on screw-in BBs and two front derailer standards (one of which affects the effective BB length), you have to be clever.
The two frame standards are 68mm and 73mm. The difference in width is 5mm.
Further, the two broad types of FD available are ones that attach to the frame seat tube via clamp or braze-on. These don't affect the BB. However, there is another type of FD called E-type. This BB has a clamp that is held onto the bike via a bracket that sits between the BB cup and frame. Have a look at the E-type FD in the photo.
View attachment 148981
That black part is a plat that's 2.5mm wide.
Shimano thus supplies the Hollowtech BB with three spacers, each 2.5mm wide, but the crank is only supplied in one length which is the longest required for a 73mm BB and an E-type derailer.
The configuration is thus like this:
73mm BB with E-type FD - no spacers.
73mm with other (frame clamp type) FD - one spacer on the right (where the E-type FD would have been).
68mm with E-type - one spacer on the left and one on the right.
68mm without E-type - one spacer on the left and two on the right.
If you add up the 2.5mm spacings in each case - i.e. spacers or E-type bracket, you'll always get 75.5mm, which is the effective length of the axle on a Hollowtech crank.
Further, the location of the spacers are important so that the crank is in the middle of the bike.
In your case you have one spacer too many and your left crank's splines are thus not properly engaged and will strip out and the crank is not in the centre of the bike. Your chainline is still correct, because you have the right amount of spacers on the right, where it matters but your crank will strip, perhaps even the preload adjuster as well if you are not careful.
Edit: Thanks to the ever vigilant
@Ajax Bay, my unofficial proof reader, I've corrected a mistake in the second option of the four possible configurations. I hope I had no-one scratching holes in their scalps whilst reading "without" when it should have been with. I've also added a clarifying bit in brackets.