The mission variance you get is based on type of mission you take. Ones I know for sure are:
1- delivery missions accepted at source - take data or cargo they give you to destination X.
*these missions never change destination on you, ever. The mission variance that is thrown in is a high probability for email that asks for faster delivery (extra cash if succeed, but no negative consequence if you don't)
the common bug here is the 0.0 time to deliver request, making achievement of the new hurry up bonus insta fail
2- sourcing missions acecpted at source - requires bringing back desired cargo from somewhere else you buy/acquire
*these are the worst destination changers, the probability is high and they change regardless of your ability to deliver. Mission variance is to change destination from where you accepted mission to other station - which may be an outpost your large ship can't land at
*these are also the worst for assigned NPC threat - another mission variance that can stack with the above destination change is an email alerting you that NPC MadSkilz1 has been sent against you, he has infinite jump range, zero cool down FSD even after failed interdiction, and can teleport from behind you as you high wake to wait for you as you land
3- smuggle contraband delivery - same as #1, fortunately the destination can't change, just time and/or assigned threat npc
4- go salvage / kill and acquire as loot / etc some desired items and bring back to station where mission was accepted -
*like #2 , the destination to bring back can change and seems to with high probability
the short rule of thumb seems to be if you take a mission from X to go to Y, your destination never changed, just other random variables like time, threat NPCs assigned to hunt you, etc
but all missions which require you to come back to the station you accepted mission from, can and will change destination. The only time you can guarantee the new destination with be very close to original station is if you do long range missions outside the bubble, where the new changed destination will always it seems be the closest station/system to mahatma outside the bubble source station (e.g. Sothis gets changed to CEOS, or vice versa, but doesn't thank god change sothis to lembava.
