Are you using a tip on the syringe, or just sticking it in your solution? If you're using a blunt tip or similar, are you accounting for the solution in the tip? The marks on the syringe don't factor that, and at the high concentration / small doses you're using I could see it adding up. At 10mg per mL you're only measuring out .05 to .075 mL for a single dose, and a small bit of solution on the outside of your syringe tip right now can be an extra 25-50% over what you planned.

At 60+ doses per month, it only takes a little bit of innacuracy in each of your measurements to throw that all off, and you could be dosing .1 mL or more. I'd personally change the solution's ratio to more like 25mg per 10mL, putting your usual dose at .2 to .3 mL. It's still a manageable amount of liquid, and tiny errors won't accidentally double your dose.

Of course, if you already factor all of that in and are still using ~40% more than you expect, it's probably what everyone else says - roommate or someone else is helping themselves.