When I purchasaed my particular WM, the Highlite, it was rated at 40 degrees. A couple of years later, they changed it to 35 degrees, but specs remained identical.

So maybe you'd be right..Let's say they have "accurate" ratings, which by the lights of some manufacturers, would be conservative.

I actually think that's part of their marketing plan. That is, a WM bag rated for a given temperature is intended by the company to be warmer than a Brand X bag rated for same temperature.

I'm only guessing that's the case, but if so, that's not a bad or unwise goal for them to have, given their prices and relatively small piece of the market. Sterling reputation is necessary for premium price.

Also, the water-repellency of a newly purchased sleeping bag shell may not be much reason to pay nearly twice the going rate for a sleeping bag or whatever it amounts to. At least not for me, mostly using a bivvy cover, partly to protect my investment.