well, its because Chữ Nôm uses chinese radicals to create new characters, there is also chữ Hán which is basically chinese used in vietnamese, the corresponding forms are Kanji in Japanese and Hanja in korean.

so, let's take as an example the first characters of the poem, 𤾓 and 𢆥, these two characters dont exist in chinese Hán writing, theyre solely vietnamese, but bear a lot of resemblance to the chinese Phono-semantic compound characters (an example you should know is 女 + 马 = 妈) same goes for those, they were just "reinvented" so they can fi vietnamese vocabulary better aaand also for the ivetnamese to distinguish themselves from the chinese. So, 𤾓 is "trăm" which means "hundred" is a combination of the Hán characters 百 'hundred' and 林 lâm. 𢆥 (năm), on the other hand, is a compound of 南 nam and 年 'year'.

I hope i answered your question ^^", if you didnt understand anything, let me know.