Perhaps due to the increase in the TSA tax on July 21st? If you bought before then, the TSA tax would have been $5 for a non-stop roundtrip ($2.50/segment). Tickets issued after July 21st would be $11.20 for non-stop roundtrip ($5.60/each way). So perhaps doing a reissue triggers the higher tax on the entire ticket ($6.20 higher for a roundtrip). Maybe DL is rounding down to $6 and eating the $.20.