Why does my c-lightning node pay more than the bill plus routing fees?

In your c-lightning node, you have configured your maximum rate which is used by the Pay order to make the payment.

The BOLT04 protocol explains how the protocol can overpay up to twice the amount needed to hide payments in order to improve data privacy.

if the amount paid is more than twice the amount expected:
SHOULD fail the HTLC.

SHOULD return an error incorrect_or_unknown_payment_details.

Note: This allows the original node to reduce information leakage by changing the amount without allowing a gross overpayment.

In combination with this merged extraction request, c-lightning has solved this problem.
implementation of random overpayments up to the maximum commission rate.

So, usually c-lightning will randomly pay your transaction with an amount less than the maximum you are willing to pay.

Not that in the past, Ind had not accepted the overpayments because they did not seem to have implemented this part of the protocol.