We were booking some seats but did it over the span of a few weeks.

The first time we booked seats on Legs 1, 2 & 4
And it looks fine.

The second time we booked seats on 3 & made a change to 4 going from a regular seat to a premium seat.
This time it said the total would be $250 but only billed the credit card $130
Then we received a second email titled Residual voucher with the following text:

You have paid for a flight or services with a voucher. In this transaction your voucher exceeded the total amount to be paid and as such we have issued a new voucher for the remaining value. Please save your new voucher code. You may need to use this code in your correspondence with KLM and our partners.

Name: Mrs Sinecure
Number: xxx-xxxxxxxxxx
New voucher: CAD 136.00

Any one know what this residual voucher is? Is it a 2nd credit card charge? (nothing has posted yet) $130+$136 doesn't equal $250

Or is this so strange that we'll need to ask KLM directly?

Thanks,
Sinecure