uhm

Probably.

To hide the fact a child was more powerful, families began hiding thier silver haired children by dyeing. So all siblings would be treated equal. But this, over time, in turn perverse that silver haired children were inferior and thus sacrificed to give the blonde children their 'true full powers'. (As if slivered hair was a storage box)

The mistake; other than being wrong about the whole kill a child to give it to a worthy child. They noticed that the power level a child with silver hair had, and then once sacrificed. Did not match the amount of power gained by the blonde child.

To put it simply if a silver haired child is a 9 and a blonde child is a 1.
1+9=10
However, their math was resulting in;
1+9=8
Because somehow in the equation only 8 of the 9 a silver hair child possessed was transferred over to the blonde.
Where did that 1 go?
It went to the pit/tower in which the children were sacrificed. Speculation that they were so filled with anger, fear, and a want to live- each sacrifice child held back a piece of their magic. This was going on for decades, probably 100 children. And then came Leslie, who was able to escape the fate they all wished they had, allowing her to absorb the power they have been holding onto themselves...

Hence why I believe Lesie is probably very OP as you say.

Note:
I've never read the novel or looked for spoilers, this is purely all I've deduced from the actual 92 chapters so far. I am no expert and obviously not the author and as a human could have understood it wrong, however I do feel confident in my understanding.

Woo nice explanation,thanks

thanks for the awesome explanation it’s way more clear now XD translators should let u handle the explaining LOL jk XD i kno they tried their besttt XD