The -1, 0, +1 stuff means a shift of the end value of a compound.
For example:
http://wlodb.com/compounds?checked=yes&sear...
Skeleton Steel Sword is a +3 Primary because:
The average rank of the ingredients is 18 (rounded down), and skeleton steel sword has a rank of 21. So, 21 is 3 more than 18, making it a +3 compound.
And it's a +3 Primary because it was done in primary alchemy. Not matter the alchemy you use, the same -1, 0, +1 thing applies.

Also, the higher the rank increase the rarer it is. Without using an alchemy book, +4 is the maximum for Primary and Junior Alchemy. +5 for Superior Alchemy from what I've been told.