Difference between explanation and clarificatioin

Dear colleagues,

Please help me to clarify can I use the word " Explanation" like please provide the explanation why this bill is higher than the approval amount.

The issue raised while I approving the bills from the CIVIL department that they Sent one Bill which is 50% lower than the contract amount and mentioned on the bill that it is final and contract was finished.

I send a mail that to provide the explanation why their is huge variance between the Final bill and the Contract value.

I have send the mail to head of the CIVIL. He says that As Iam junior to him I should not suppose to use the word "explanation" instead I should use "clarification".  My designation is Manager Finance he is Head -CIVIL Department.

For a while I got doubt.  Then I told him the word" explanation" is more formal than “clarification". But still he didn’t agreed with me. He  feels that  I had insulted him by asking explanation.

Therefore any one please tell which word should be used. In my personal opinion both are synopsis for each other.

And English Language is not made designation wise .

Thanks

Suresh sambu