Why is a new instance of MATLAB opened when I double-click on a MATLAB

I have full administrative privileges on my Windows 8 machine. I then open a Windows Explorer and double-click on a MATLAB file. I expect the MATLAB file to open in the existing instance of MATLAB, instead another instance of MATLAB is launched, and the MATLAB file opens there in the Editor. If I double-click on another MATLAB file in Windows Explorer, it opens in Editor in the second instance of MATLAB. However, if I close the second instance of MATLAB and then double-click on a MATLAB file in Windows Explorer it will open another MATLAB instance. I am using MATLAB 2015a.

[Joseph Cheng]

is there multiple versions or installs of matlab installed on the computer?

if you start fresh with no matlab instances open and double click on a file so it opens instance 1 then open another file does it open the file in instance 1 or start off another instance.