Wyatt Earp made the right connections. Your Galco holster is designed to be tensioned by the belt, pulling the holstered handgun into the body. This applies tension to the handgun and increases retention in the holster. All good things!
There will be a break-in period with any new and properly formed holster. Holsters should be fitted tightly enough to prevent movement of the handgun inside the holster (the primary cause of holster wear) as well as to prevent the handgun from sliding out on its own. After a period of use (a few weeks) the leather will start to become more supple. If not well-fitted to the handgun, the holster will start to become sloppy in fit within a relatively short time.
A solidly made belt of the correct size to fit the holster slots/belt loops is far more important than most folks realize. Narrow belts, and belts made of lightweight materials, must be tightened to an uncomfortable degree to secure a holstered handgun of any significant weight.
Most of the holsters that I make (when new) can be held upside down with the handgun in them without releasing the handgun. After a reasonable break-in period, I expect the handgun to come out smoothly without undue resistance, while being well retained in normal carry.
Interesting that a holster formed to the SW99 fits better than one made for your M&P. Some makers use a generic molding piece, fitting one holster to several handguns. That is an idea that I really dislike; my holsters are hand-fitted to the specific handguns they are intended to carry. Anything less is just second-rate (in my humble opinion, after 37 years of making holsters). Galco makes a fine product, so don't take this as a slam against that company.