An antenna that's the wrong length doesn't stop being an antenna; it just becomes a less efficient antenna for the given frequency.
As a matter of fact, basically every conductor is an antenna. That's why your antenna cable cannot just be a single wire running from your RTL-SDR to the antenna; it would become part of the antenna and pick up signals, as well.
As cheap and insensitive an RTL-SDR is, compared to say your phone for cellular frequencies it's designed for, you seem to be picking up a lot of the 40 m transmission – that happens, though you could expect quite significant attenuation.
Now, it does also happen that transmissions on some other bands actually get mixed up in frequency, before they reach your antenna. But then you wouldn't "see" them in the 40m band, but somewhere else!
The cause for this frequency mixing would be a defect either with the transmitter, or a large antenna structure close to you, introducing nonlinearities. In the case of active components like transmitter power amplifiers, that is something you always need to take care about when designing the transmitter, and add sufficient filtering to suppress such harmonics (or else you'd be in violation of your transmitting license). In the case of passive components, like cabling, filters, and antennas themselves, there's a thing called Passive Intermodulation Products, and they happen when you either put too strong a field across a material that starts to behave non-linearily, or when corrosion on a metal connector lead to semiconductor properties. Both annoying!