In a set totalled at 8 (8 here may refer to 八方 as suggested by the following phrase:上下四维..) , subtracting 1 element from it, 7 are left. So, losing 1 in some way is equivalent to gainning 7.
It is all relative. Don't get obsessed with the notion of losing 1.
I guess this COULD be what this line is suggesting.
But I wouldn't get worried about its "true" meaning, otherwise the monk who wrote this might be embarrassed.
