What is it mean Difficuly & prob when using vanitygen? I am running this command

$ vanitygen64.exe 1abcdefg
Difficulty: 778316249440
[125.17 Kkey/s][total 39015067414][Prob 4.9%][50% in 46.3d]



What is it mean Difficulty? I understand that higher takes more time, but what is the scale, why it is different from total? Why alphabet is more difficult than only digits vanitygen1234567`?
What is it mean 50% in 46 days, why it not tell me 100%. After 46 days, how much percent will be that it will complete in the next 46 days? If it is same reasons  as in Half-Life in nuclear physics I understand it.
The big question. What it is Prob 4.9%? Prob of what?
Difficulty tells you how likely it is that vanitygen will be able to find the address pattern you're looking for on the next guess. A value of 2n is twice as difficult (half as likely) than a value of n.
Because the process is completely random, it's impossible to give you a '100%' ETA. We can only estimate that, on average, you will find your key before some time period half the time (hence why 50% is used). Note that it re-adjusts the value to 75% once it hits that 50% mark (and re-adjusts again after hitting 75%, until the key is eventually found).
That is the likelihood that you would have found your key by now, from the time when you started generating your key.