For many months I thought the HQ % meant that probability with each hit at the node. Alas, it really means your overall chance of getting HQ for all hits combined. To achieve this effect SE actually lowers your % chance after you are successful. In effect, the % is a soft cap.

Take the example of 67% HQ chance. With 4 hits the chance that all 4 hits will be successful is, statistically .67*.67*.67*.67 = 20%. Now, not only do I not see every 5th node yield HQ on every hit I have NEVER seen it happen at all! This means that SE reduces your next hit chance after a success.

For example, if your HQ chance is 41% and you got HQ on the first of 4 hits, your chance of getting HQ is now effectively reduced to 16%. Get a second HQ success and your chance is now reduced to near 0% for the last two. I say near zero because I have actually seen my botanist get 4 HQ hits (a couple times). But never on my miner.

It would be nice if SE would update the HQ % to be real-time as the node progresses.