It might be related to "harmonics" in the spring.

Anyone familiar with "spring reverbs" in guitar amplifiers will get the picture quickly.

Here's how it was explained to me:

In something like a Sten, the "return spring", in launching the bolt forward, "uncompresses" to provide the energy to propel the bolt. Like a tubular chime, it will have a fundamental "frequency", closely related to its overall length, and several harmonic frequencies. All of these will be triggered, so to speak, when the sear suddenly releases the bolt. In the time that it takes for the bolt to run forward, strip a round from the magazine and chamber it, there will have been several of these "harmonic" waves traveling up and down the spring. If a "compression front" of one of these waves arrives at the bolt at the instant of cartridge pickup, it will reinforce the forward motion / energy.
If the harmonic wave has reached the bolt and started to reflect, it will subtract from the deliverable energy at that moment.

Not complete cancellation, but essentially partial wave cancellation.

Given that second, (even / "nice" harmonic) and third ("ugly") harmonic and so on, are happening, maybe the best idea is to have a return spring somewhat more "butch" than theoretically required.

And that is just for "single shot" from an open bolt. It probably gets more interesting when the switch goes to "FUN".

"Spring-bind" is a bad thing all round in either case.