This is a common question about the the MTU K.
K5 (MTU) is not used in the "EIGRP cost/metric calculation", but it is used as a tie-breaker.
Here is a straight-forward explanation: EIGRP MTU Metric - Ivan Pepelnjak
"Every so often I get a question about the MTU metric in EIGRP and
whether it’s used at all or not. It actually is: if your router would
have to ignore some equal-cost paths to the same destination (the number
of equal-cost paths exceeds the value of the maximum-paths router configuration parameter), it ignores those with the lowest MTU metric.....<more>"