If you're a runner in the Midwest, spring means finally getting off the treadmill and onto the roads. Will your treadmill pace directly carry over? No, running on a treadmill and running outside just isn't the same.
Tire engineers have to deal with a similar issue as a tire moves from the design phase to real-world use. When a tire engineer works on an all-season tire, he or she is forced to balance driver needs because the tread pattern, rubber compound and tread depth most conducive to snow traction is diametrically opposed to what's needed for the best dry grip. That's why having dedicated winter and summer tires always gets you the best performance in each season.
Even with due diligence, it's exceedingly difficult to create an all-season tire with the "right" balance of wet, dry and snow performance. When Michelin launched the Pilot Sport A/S 3, we were super impressed by it's wet and dry grip relative to its predecessor, the Pilot Super Sport A/S Plus. However, snow testing and review feedback found a marked decrease in snow performance. For certain snowbelt drivers, the Pilot Sport A/S 3 had the "wrong" mix of performance characteristics, therefore Michelin went back to the drawing board to create the Pilot Sport A/S 3+.
Michelin's internal testing indicates a 28% improvement in snow traction compared to the Pilot Sport A/S 3. From their standpoint, there isn't a substantive drop-off in wet or dry performance.