no check is accurate to a monster's stats

for example, Undyne the Undying has 12 ATK and 5 DEF, even though her check says she has 99 ATK and DEF, this is because if her stats were accurate, you'd die in 1 hit

likewise, Mettaton NEO's stats are also wrong (i don't remember what his actual ATK is), he has -40k DEF to make sure he dies in one hit, as he has more HP than Undyne the Undying (Undyne has 23k HP while Mettaton NEO has 30k)

Asriel's stats are said to be infinite, but he actually has a specific amount of ATK (which i also don't remember) and 9999 DEF, which means that even an attack from the Real Knife will not hurt him

the only monster that has stats accurate to their check is Sans, who does indeed only have 1 ATK and DEF (Altough neither matter since he dodges attacks and deals 1 damage with no immunity frames)