Ionisation enthalpies of $Ce$,$Pr$ and $Nd$ are higher than $Th$,$Pa$ and $U$. Why?

(N/A) In the beginning of the actinoid series,when $5f$-orbitals begin to be occupied,they penetrate less into the inner core of electrons compared to $4f$-orbitals in lanthanoids.
Consequently,$5f$-electrons are more effectively shielded from the nuclear charge than $4f$-electrons.
Due to this,the outer electrons in actinoids are less tightly held by the nucleus and are more easily available for chemical bonding.
Therefore,$Th$,$Pa$ and $U$ have lower ionization enthalpies compared to their lanthanoid counterparts $Ce$,$Pr$ and $Nd$.