Unreliable dynamic pricing for variable products

My client sells drill bits in several lengths. We are in the process of building a store for him. It is not public yet. I can give you the login information through a private message. For the smaller bits, he wants to offer 10% discount when you buy 12 bits or more and for the larger ones, he wants to offer 10% discount when you buy at least 6.
What we have seen, is that only for the smaller 200 series in metal drilling, this doesn’t work properly.You get the discount when you order 9 already instead of when you order 12. I looked at the rules my client implemented in your plugin, and I don’t see where the problem is. There are similar rules for the 300 series and that seems to work fine so far.
The website is hosted with Ipower with their WP essentials hosting package. Please let me know how I can provide you with the login info privately. I will also show you the exact steps if needed.

The page I need help with: [log in to see the link]