It measures the time to fill the current asio guard buffer in relation to the asio guard buffer size. If the asio guard buffer is 10ms (to keep it simple) and it takes cubase 5ms to calculate that buffer, it will read 50%. If it takes 2ms 20% if it takes 11ms you will get an overload and a big dropout. It is that simple. Asio guard load should give the same results in C12 and C14. If not there can be settings involved like suspend vst3 plugins with no audio (which lowers the load) or if there is a track record enabled. Also cubase might spread the load differently in different versions, but you would have to load a project with many many plugins and a very high load like 90% to see if one or the other is more efficient.