Why ionisation enthalpy of early lanthanides are expected to be greater than early actinides?
This data is incorrct, and I am saying so because the Ionisation energy of the actinoid is a decent bit higher than the corresponding lanthanide

In each case IE 1 is significantly higher for lanthanoids, IE 2 is still higher for lanthanoids and IE 3 is comparable. 

$\ce{Ce}$ $534.4$, $1050$, $1949$

$\ce{Th}$ $587$, $1110$, $1930$

These were two examples where i showed all 3 IEs

For, the sake of further clarification, here is a the IE1 of the first few 

$\ce{Pr}$ $527$, $\ce{Pa}$ $568$

$\ce{Nd}$ $533.1$, $\ce{U}$ $597.1$

$\ce{Pm}$ $540$, $\ce{Np}$ $604.5$ 

$\ce{Sm}$ $544.5$, $\ce{Pu}$ $584.7$

(units used are $kJ/mol$)

The reason you stated in your doubt is absolutely correct, as even if we were to conclude at these aren't accurately determined, we see this phenomenon elsewhere in the periodic table also, where the values are accurately determined. 


$\ce{Sn}$ is the lowest IE element of GP 14 and not $\ce{Pb}$.
$\ce{Fr}$ has a higher IE than $\ce{Cs}$.
$\ce{Ra}$ has a higher IE than $\ce{Ba}$.


All due to the same reason poor shieling, which increases the effective nuclear charge, which increases nuclear attraction, which in turn leads to a higher IE.

References

Wikipedia data page