Why do you think Beowulf boasts of his accomplishments in lines 228-239?

In lines 228-239, Beowulf does not boast of his accomplishments, instead, he is expressing modesty in a speech before a battle with Grendel's Mother. However, later in the epic (from line 1290 and beyond in Seamus Heaney's translation)—and before he goes back on his own to fight the dragon that ravaged his kingdom—Beowulf does offer a proud self-reflection on his past as he prepares himself for another life-or-death duel.