When defending the BB your goal is to continue with hands that
lose less than 1bb.
For example, here's a BB defend vs UTG open, with the EV's shown. KTs has 0.3 EV
relative to folding. That means it expects to lose 0.7bb, which is still higher EV than folding your blind (-1 bb). So if you measure that with a HUD, you might think it's a losing call. But it's still higher EV than folding.
Here's BB defend vs SB open. There are many more hands that are "losing, but still higher EV than folding", which may explain your results.
Some people have this idea that "folding is always 0 EV", but that's misleading. We only define it that way because it makes the math easier. Folding loses whatever you put into the pot. The truth is that poker is full of spots where you're deciding between multiple losing options and trying to find the option that loses the least.
You can view these
charts for free if you want to study them more.