Ionization enthalpies of Transition elements

“There is generally expected increasing trend in the values as the effective nuclear charge increases. **However** the value[First ionisation enthalpy] of Cr is lower because of the absence of regarding any change in d configuration and the value of Zn higher because it represents an ionisation from 4s level” 


For, Cr it is 653 kJ/mol and for Zn it is 906 kJ/mol


It seems that the statement regarding Zn undoes what is pointed out by the statement regarding Cr, as for Zn it changes from 4s2 3d10 to 4s1 3d10 and for transition from Cr to Cr +1 the electronic configuration changes from 4s1 3d5 to 3d5, clearly both have change in their 4s configuration and not in 3d configuration.


My first question is what does this mean?
Problem in understanding this is that the book has not mentioned anything about filled 4s2 configuration of Zn, it seems that it is supposed to be understood from the statement alone.


Even after ignoring that, if I think in terms of effective nuclear charge, which is higher for Zn and as a result higher energy required for ionisation of electron from 4s2, but the problem in that that it does not seem to be the explanation of that because of that **However** , which seems to point to some exception.


I have checked other questions on this site regarding the irregular trend of ionisation enthalpy for transition element but they don’t seem to me to address this.


