Have you replaced the tub spout in your bathroom? The reason I ask is that chinese manufacturers will make spouts that LOOK identical to the Moen or Delta spouts, but won't have the same closing action that true Moen or Delta spouts do.
For example, Moen makes a slip-on spout called the Model #3931. It has the standard plunger style gate that most diverter spouts do. The difference is that the #3931 will have something called a "cup washer" in that gate that "inflates with water" when it's pushed in front of the end of the copper pipe the spout slips onto. When that cup washer inflates, the front of it moves forward to seal against the end of the pipe so that no, nada, zero, zilch water leaks out the spout when the shower is on. Delta and Ondine have closure mechanisms that work on a pressure principle just like the water mixing valve on a washing machine, and neither should lose any water when the shower is on either.
I'm pretty sure that if you phone up some of the plumbing wholesalers in your area and find out who the Delta sales rep or agency is in your area, they'll have Delta spouts on display so that you can compare yours to. I expect yours is a cheap spout made in China that's intended to merely look the same as a real Delta diverter spout. Home centers will sell these offshore knock offs because they cost a lot less than the real thing, but they just don't work as well as the real thing.
In my case, it's important for me to have good shower performance in my bathrooms because people do want that. I have installed Niagara N2130 shower heads (which won an award for excellence in design) because they provide the best shower performance while keeping water usage low and Moen 3931 diverter spouts (which use that "cup washer" design) in all of my apartments. Since I made that switch, not a single tenant has complained about the performance of the shower in their bathroom.
Moen also makes screw on diverter spouts with the cup washer in the gate, and you can get a repair kit that includes a new cup washer, gate and plunger to rebuild your Moen diverter spout if the cup washer is ever damaged and starts leaking water. Just ask for Moen Part #10644. They're supposed to cost $12 each (almost as much as a new #3931 spout, but if you tell Moen that your spout is leaking, they'll send you a repair kit free.
The other important advantage of the Moen #3931 slip on diverter spout is that it doesn't have a set screw that impinges on the copper pipe. Instead it has a "yoke" that clamps around the pipe, thereby distributing the gripping force over a much larger area of the pipe, and therefore it's less likely to damage the copper pipe.