So if you see the BLAST parameters it says
The Expected value E is a parameter that describes the number of hits one can "expect" to get by chance when searching a database of particular size. It decreases exponentially as the score (S) increases. Essential, the E value describes the randomness background noise. For example an E value of 1 assigned to a hit can be interpreted as meaning that a database of the current size one might expect to see 1 match with a similar score simply by chance. Lower the E value the better."
How could there be something "BY CHANCE" it's a software right ?