The -10deg is for tensioning after belt installation. Oftentimes the belt will not fit exactly with both marks dead on, especially with a new belt that hasn't stretched yet. There can also be slight manufacturing variations in the engine parts themselves. The stock cam gear timing mark-to-keyway relationship can vary as much as 2 deg ( see here ->
http://www.lindseyracing.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LR&Category_Code=944CAMGEAR ). As long as it's very close, like within half a tooth, it should be fine. I've heard it can be set off a tooth one way or the other and not damage anything, but not much more than that.