This is the explanation from Bread World (maintained by ACH, the parent company of Fleischmann's) and the one I would trust:
"RapidRise and Bread Machine Yeast are different strains than Active Dry Yeast. RapidRise and Bread Machine Yeast are grown with a higher level of nutrients and are dried to lower moisture content. The particle size of RapidRise and Bread Machine Yeast are finely granulated to allow complete hydration of the yeast cells during the mixing process. The Active Dry Yeast larger particle size should be dissolved in water to achieve complete hydration prior to adding to the mixer. In addition, RapidRise and Bread Machine Yeast contain ascorbic acid resulting in increased loaf volumes."
The bottom line is that active dry yeast should always be hydrated in water before use, while RapidRise and Bread Machine Yeast can be mixed straight into the dough.
I find this interesting information. I've been making all our household's bread products for most of my life and have only, except in the early days, used active dry yeast...for every application. Bread machine or not. Plus, I've never hydrated my yeast before using it, with the minor exception if I'm not sure how old my yeast is. That, too, usually isn't a problem because I use it up so quickly.
I buy yeast in one-pound vacuum-sealed packages. When I open one, the yeast goes immediately into a glass jar with a tightly-sealing lid and into the freezer. Annually, I estimate we use about 4 pounds of yeast. But that's a conservative estimate.
I mentioned the "early days," which is when I used the cake yeast, found in the refrigerated section of my markets. Now, it's nearly impossible to find it in my region and when I see it, it's outrageously expensive. About $1.50 per cake. I can buy a LOT of dry yeast for several of those cakes. Plus, I've noticed that the three-section packs of dry yeast are pretty pricey as well.
My brother, who is an awesome cook and baker, swears he can't make any yeast goods using dry yeast. Claims he can't get it to rise. I've walked him through all sorts of steps and he still insists the dry won't work for him. As a result, last Christmas I gave him a half dozen cakes of yeast as a present.
By now, I've memorized the equivalent of a packet of dry yeast (2 1/4 teaspoons) so I can use my bulk yeast just as easily as a packet or a fraction of a packet if I'm reducing a recipe.