The "every x ..." is a loop. It triggers the event on every 10000ms in your case. So the event will be triggered on 0ms, 10000ms, 20000ms, etc.
Combinig this with trigger once is odd, but what at least happens is that the event is triggered first on 0ms.
If you just want to make the layer visible after 10 seconds of the layouts playing time has passed, system compare against the timer:
+Timer greater than or equal 10000
I wouldn't use "Timer equal to", because it is not guaranteed that you will catch the exact moment of 10000ms