Garbage Collector does not release memory on his own

Why does my garbage collector(G1 GC) do not release memory altough he could?The Used Heap is going up over time, the step fall at the end is because I forced the GC to release memory through jcmd <pid> GC.run

Why is that? Is that normal behaviour?

I started the GC with these parameter -Xms64m -Xmx256m -XX:G1ReservePercent=50