In Maseches Pesachim 57b, the last page of of the fourth chapter ends abruptly halfway through the page, followed by "הדרן עלך מקום שנהגו וסליקא לה פסח ראשון." The rest of the page is filled by a large paragraph titled פיסקי הלכות פסח ראשון בקצרה.
The same thing appears later on Maseches Pesachim 99a at the end of the ninth chapter, this time saying "הדרן אליך פסח שני" and the following paragraph titled פיסקי הלכות פסח שני בקצרה.
Why does the hadran at the end of these chapters make it seem as though a masechta was completed? Why is it then followed by this summary paragraph, unlike anywhere else in shas (that I am aware of)? Do we know who wrote this paragraph? (This sounds like multiple questions but I feel like they probably have the same or related answers)