A lot of times the models on Purpleport and Modelmayhem type sites will state modest posing levels for a number of reasons;
These sites attract a lot of photographers with no real past history, and for safety reasons they will often quote lower levels, and insists on some type of chaperone. Just think how vulnerable you'd feel meeting a complete stranger alone, often in an unknown location, and then stripping off totally naked and posing in explicit positions ...
The models know that these sites are easily searchable by anyone, including friends, family, and possible future employers. This being the case they want to have some control over what's being shown, stating lower working levels obviously achieves this.
Some models once they get to know and trust a photographer will do private work at higher levels, but this is on the assumption that the results aren't published and/or sold/shared with a third party.
With the advent of paywall sites, they realise that they can substantially boost their modelling income by keeping more explicit work on these sites and by using a nom de plume.
It cannot be assumed that just because a model poses to art-nude levels (akin to posing for a life drawing class) that they must go beyond this level. Not all women want pictures of a 10" dildo up their Jack & Danny spread all over the internet .....
Of course, most people have a favourite model or two that they've only ever seen posing at more modest levels that they'd like to see more of in terms of risqué posing. But the reality of life is that sometimes these thoughts will only ever remain a wishful dream!