Easy mate,
In situations where folk are running more and more load on teh actuator by going shorter and shorter, you often finmd that under concentrated load when the wastegate is asked to bypass far more volume of air per second.. IE.. 5K in 5th.. the gate will no longer open wide enough as youve removed all its travel capability.

In this scenario, the boost will start to climb higher and higher as the revs go up and up simply because if teh gate doesnt open enough, then the extra air will be FORCED to flow through the scroll into teh turbine and of course spin it harder. This is only normally noticeable in 4th and 5th at high RPM.

Hope this helps... be carefull out there kids