I have a question about the weight limits for glass shelving.?

I've had a display cabinet in which I've been displaying model cars for the past 20 years.  I've decided to add another shelf, but, after reading a few websites, am beginning to wonder if I've been WAY overloading these.  The shelves are 1/4" glass, 12 1/2" deep and span 90 1/2", with a support in the middle (45 1/4").  I haven't a clue if they're tempered, but I'm guessing they are not. The cars I display are not small and usually weigh 2-3 pounds each. On any particular row, I display about 17 cars (with one usually right on the center support).  So, that's eight on each span, at almost three pounds each.  

I ran some of the calculations on various sites and, apparently, I'm only supposed to put seven pounds on the span.  How in the world are supporting 24 pounds???   Again, I've had no issues for almost 20 years, but now I'm wondering if this is an accident waiting to happen.   I can add some more supports, but I just wanted some thoughts on why this hasn't catastrophically failed already.

I've included a photo to show you the current setup.