Going from a victory with 21/71 remaining to a loss with 61/92 of the enemy remaining is not an acceptable chance, but I'll make more proper tests when I have the time (to see if the changing of the speed has an impact on average or variance or not enough to tell).
I think it makes perfect sense if the bad luck was in the beginning of the fight. If in the beginning of the fight one side under performs by chance, while the other side over performs, you get a snowball effect. Say for example two forces meet, one has twice the number of ships but each shot does only half the damage. Without randomness you get a draw as both sides have the same strength. But if you introduce a random factor things change. Suddenly you can get statistical flukes like one side missing all shots of their opening salvo while the other side doesn't miss a single one. That may be very unlikely but the nature of chance permits it. Other things can also skew the result, like too many ships targeting a single ship thus wasting some of the potential damage.
The smaller force being struck by bad luck has a more severe effect than the other way around, because each shot represents a higher percentage of the potential damage. If the balance is altered by chance early in the fight, each successive round of fighting will alter the balance increasingly more in favour of the winning side.