Windows 7: Taskbar group don't always show when clicked

When I click on a group in the taskbar, it usually shows a preview of all the instances of this program. But sometimes, about 5% of the time, it shows the same menu as when I click on it with the right mouse button (recents, start new, pin/unpin, close all). I have no idea of the conditions leading to this different behavior. I click on the group the same way as usual, with the left mouse button.

This is really annoying because I sometimes close all windows or start a new program when the wrong menu appears!



Any idea why it happens? Anyone is having the same issue?

Thanks!