In some cases (C4, C2 and H1, here), the rearrangement of the product can form a film distributed more evenly than before irradiation, leading to an artificial increase of absorbance values. With protocol 1, this results in a photostability percentage that is higher than that found by HPLC, and which can be more than 100%.
In other cases (C3, C7 and C6, here), rearrangement can form a film distributed less evenly than before irradiation, resulting in lower artificial absorbance values than with protocol 1, and a photostability percentage lower than that found by HPLC.
It is possible the formula self-levels due to certain conditions such as UV and/or thermal exposure or other raw materials in the formula. The film product can slip to the deeper valleys of the PMMA plate, leaving the upper peak unprotected. Conversely, a formula may tighten or tense itself and cover the valleys or peaks of the PMMA plate more equally.