[HumJanenge] ADVICE: Computation of inter-se seniority beyween IAS and IPS

In view of the media reports that DoPT apparently informed Sushma
Swaraj (LoP) that Satyanand Mishra is the seniormost IC, can someone
expert in babudom rules/circulars explain why IC(SM) (IAS:73) is more
senior that IC(MLS) (IPS:72).

(Disclaimer: This is just an academic query and does not reflect a
preference for either of them)

I believe there are seniority computation rules/regulations under the
All-India Services Act, but these are service wise. How do these
compare apples versus oranges, when the apples write the rules ?

Sarbajit