The water comes out of both the shower head and the tab simultaneously. If the knob controlling the water flow is turned to the side that control the water to shower head then about 75% of the water flows to shower head and at the same time about 25% flows to the tab. I can't figure out why?

I will have to assume this issue developed and was not present at first use after the initial installation.

There are generally only two reasons for your problem. The first is that the diverter mechanism is simply wearing out and/or broken. The second it that there is a foreign object preventing it from doing its job correctly. Either way the diverter must come out. I have provided a couple of links below that should help get it out.

Once out, put a wadded up towel over the end of the tub spout opening so as to prevent any water from coming out of it. Then, for just a moment, turn your shower on full and adjust your temperature from full cold to full hot. This process should dislodge any foreign matter and send it out the opening the diverter came out of and also will probably get you wet. I usually plug the tub so I can catch what, if anything, comes out. Having a second person holding a bowl over the diverter opening helps reduce the water splash.

Rinse off the diverter and reinstall it. Turn the shower on and see if there is any difference in how much water comes out the spout while in the shower mode. If nothing has changed then the next step is to replace the diverter.

The diverter assembly part number is A603819

http://www.danze.com/products/pdfs/D113000_ip.pdf

http://www.danze.com/products/pdfs/D500058T_ip.pdf