Because the rear are wider. To keep the same circumstance the percent of width would be different. Tire size is tread width, the percent of tread width, and rim size. Example (245x40x17) 245 is tread width, 40 is 40% of 245 and the rim is 17. Hope this helps.
Gbarringer100's info is correct.
The wheel/tyre diameter should be the the same front and rear ie. the sidewall height should be the same at 100ish mm.
So for the front the sidewall height is 225/100x40 = 101.25mm, for rear 245/100x40 = 98mm which is near enough. If you have 45's on the rear the sidewall height will increase to 110mm increasing the overall wheel diameter by 20mm compared to the front.
If it could cause ABS/ESP/TPMS errors it's most likely do so if the fronts were worn and the rears were new, this would make for the largest differential in diameters.
Here's the sizes for 16/17/18 and 19 rims for the slk350.
Front : 205/55Z16 | Rear : 225/50Z16
Front : 225/45W17 | Rear : 245/40W17
Front : 225/40Y18 | Rear : 245/35Y18
Front : 225/35Y19 | Rear : 255/30Y19
Personally I wouldn't worry about it unless it did cause ABS type errors which I doubt it will. Handling may feel a bit different as the centre of gravity will be higher on the rear and the tyres will be a little more 'squishy' but it's probably going to be a marginal change.