I've never heard of an official saddle fitting (which doesn't mean they aren't done). Everybody pretty much just gets on and decides if it's comfortable or uncomfortable, then whines about it in forums... But I haven't ridden anything but Brooks B-17s in years, and they break in, so your "sit bones" fit into the little dimples they form in the saddle. Plastic saddles don't do that, so maybe there's something to buying one that matches your butt width.
How did they determine what width you needed? Measure between the bones, or what?
I secretly suspect this is a bogus issue, but if it's not, 40mm would certainly make a difference. That's more than an inch and a half on something that's typically six inches wide--that's a significant fraction. And 187 is a WIDE saddle. Often those will feel good in the shop, but start to hurt after you ride a few miles. Even my B-17s are only 170mm.