You're not lucky---well, you might be lucky in many other ways :), but just not in this particular way. The noise reduction you experience is intentional.
Here's the basic idea: The coils in your pickups "pick up" noise in addition to the vibration of the strings. Everyone's pickups do. When you have two pickups selected at once, the signal you hear is the sum of the signals from both pickups. If the pickups are wound in opposite directions and with opposite polarity, then when their signals are combined, the noise they picked up will be 180o out of phase with each other and will therefore cancel each other out. It's exactly analogous to the idea that 1 + (-1) = 0.
The unique sound you hear from having the two pickups selected is also due to phase cancelation. Suppose you have the neck + middle pickups selected. The middle pickup, because it's closer to the bridge, will pick up slightly more upper harmonic overtones than the neck pickup. In other words, the signals coming from each pickup aren't exactly the same (which accounts for why they sound different when you select one at a time). So when combined, some of the overtones from the two pickups will be 180o out of phase and again, these will cancel each other out. This is what accounts for the "gentle lowpass filter" effect you describe.
A humbucker has two coils, reverse wound with reverse polarity, and so the combined signals of the two coils cancels out the noise just like with the neck + middle and middle + bridge selections you describe. But because the two coils in a humbucker are right next to each other, their individual signals are much closer to each other than in your guitar. The two signals from the coils in the humbucker aren't exactly the same, so the combined signal still has the phase cancelation effect. But because the two signals are so close, the frequencies cancelled tend to be very high.
Long story short: You won't get more noise cancelation from a humbucker than you currently get from your neck + middle and middle + bridge selections. In fact, if you replaced, say, your bridge pickup with a humbucker, you'd actually lose the cancelation effect from your middle + bridge selection.