good explanation to a new person , we all tend at one time or another to forget when we have been around even a few weeks/months/years that we had to ask or google how things work etc and are not all at the same technical level since some are in this for the $$$ and some the science.
Sometimes a mag drop will be due to a project for some reason or error not being included in a superblock , you can clock that @ https://gridcoinstats.eu/project and see how many were included in the latest/last superblock along with cross checking the official websites whitelist http://www.gridcoin.us/Guides/whitelist.htm .
Another reason for drops is large numbers of new users joining boinc or switching teams or new team Gridcoin joins whom have excessive hardware as todays market of video cards , you see many people with multiple 16-72 core systems and multiple gtx1030-gtx1080ti cards and those can run multiple WU per GPU depending on how you configure your plan_class or app_config.xml so it really is like playing the stock market and wall-street and lots of patience. I will tell you from my own experience my mag has been 1840 for a few days , it was in the 700s even more. I got used to 180-220 being my average with even less than my current hardware , it was nice at that time 65grc a day for a 120 mag but now 80-120 and it fluctuates all week long back and forth and is 18-22grc a day. Most users are happy with 1-3 grc a day with their quad core phone and single desktop/laptop , others expect $5USD a day.