here's the thing, say for example a Christian believe in "if saved, always saved", and that "good works is evidence of salvation", however throughout the walk that Christian struggle with doubts from time to time, doubting salvation etc.
If that Christian does not get proper assurance for salvation, they will think "for now, I have to do something, at least for now", and if practiced sufficiently and repeatedly, that in a sense becomes "work-based salvation"
and let's say that Christian know that man can never contribute to salvation, that it's all reliant on God's power
sometimes, in their walk there's spiritual dryness, or that God seem distant, they try to assure themselves some way, which goes back to "work based salvation"
or, they think up of strange theology to reconcile with those difficult times, these are possibilities ...
to help explain why there are so much confusion about salvation