Improvements in flexibility over a few weeks are usually attributed to muscles lengthening. Measurements suggest that in the short term the muscle changes very little and the nervous system changes a great deal.
Muscles resist lengthening through reflexes
Muscle contains sensory organs that detect both length and the rate of change in length, and they respond to rapid stretch by triggering contraction in the same muscle.
This reflex exists to protect the joint, and it means a fast or forceful stretch is actively opposed by the tissue being stretched.
Slow approaches allow the reflex response to subside, which is why held stretches achieve greater range than rapid ones at the same effort.
The limit reached is a tolerance limit
Studies measuring both range of motion and passive resistance find that after several weeks of stretching, range increases while measured stiffness barely moves.
What changes is how much of the sensation a person will accept before stopping. The endpoint is defined by discomfort rather than by tissue reaching its limit.
This is why range can improve within a single session, far faster than any structural remodelling of tissue could occur.
Structural change requires longer and different loading
Genuine lengthening involves adding contractile units along the muscle and remodelling the connective tissue surrounding it.
That process responds to sustained loading over months rather than to brief holds, and it appears to be driven more by loading at long muscle lengths than by passive stretching.
Strength work through a full range of motion produces flexibility gains comparable to stretching in controlled comparisons, which fits this explanation.
Range is joint-specific and structurally capped
Bone shape sets a hard limit that no amount of practice alters, and hip socket geometry in particular varies considerably between individuals.
Someone who cannot achieve a given position may be meeting bone against bone rather than encountering soft tissue restriction.
Because these differences are invisible, comparison between people in a class setting is uninformative about how anyone should progress.
Why more flexibility is not automatically better
Range that exceeds what the surrounding musculature can control leaves a joint reliant on passive structures at its end range.
People with naturally high joint mobility often gain more from strengthening through range than from further stretching.
Persistent pain during or after stretching, or joints that give way, are reasons to seek assessment rather than to continue practising through it.