Right, lore and stat-wise (you can click on their Traits and see critical immunity listed), Beasts are immune to critical hits.
This happens because the message of your critical hit success is still shown even if that shot hits a target immune to them--notice you won't see any other effect following the critical message. I guess it can be confusing due to its wording, since it no longer refers to the critical hit as being a function of your attack but rather as one of its effect ("suffers").
I think I left it in there so that you have a general idea of what your volley is capable of, seeing the true ratio of crits overall. Technically even your score sheet critical tally includes these as well. Maybe I should at least remove the message specifically for immune robots if it'll cause confusion...