This seems to be because the three marks of existence are not perceived in the nimitta which, instead of being seen as rising and falling cittas and javanas during jhana, is seen rather as something constant and one is absorbed in this nimitta.
On the counterpart sign (nimitta):
-Visuddhimagga chapter IV paragraph 31But it has neither
colour nor shape; for if it had, it would be cognizable by the eye, gross, susceptible
of comprehension [by insight] and stamped with the three
characteristics. But it is not like that.
If one could see the three marks of existence in the nimitta during jhana then the nimitta would be susceptible of comprehension by insight. But this is not how the nimitta appears and so the experience of jhana, being an experience of absorption into this nimitta, is not one in which the observing of anicca, dukkha and anatta would be possible as it is in vipassana (insight). For this reason one must exit jhana and then review the impermanence of the experience afterward, not during, if one wishes to use jhanas for insight.
Essentially, jhana is not an experience of the three characteristics of anicca, dukkha and anatta.
Or, as Richard Shankman puts it in The Experience of Samadhi (p 55):
...all experience of changing physical and mental activity ceases.
Do I understand correctly?