So you have 15 individual conductors, five each of three phases, plus any neutral and ground conductors; and for those 15 conductors, there are three bundles, each with five conductors of the same phase. Is that correct? And in one of those phases, one conductor is carrying 560A while the other four are each carrying 200A, for a total phase current on that phase of 1360A. Is that also correct? Which phase are these readings from? What are the currents in the individual conductors of the other two phases?
The cable inductance would be lower if the cables were grouped such that each group had one conductor of each phase, plus a neutral if there are also neutrals. The segregated bundles will have a higher inductance, and therefore a higher total impedance, but unless each bundle very carefully maintains its configuration within the bundle the entire run, each conductor should have nearly identical impedances, a 5% difference in cable impedance would be extremely high and no where near enough to cause what you are seeing. I would expect more difference in impedance due to differing lengths of conductors than from geometry. On the other hand, if the configuration is maintained with absolute consistency, you might get a noticeable difference between the inner edge of a bundle, where it is nearest to the bundle of an other phase than the outer edge, where there is no adjacent phase. But even that wouldn't be enough for more than a few 10s of amps difference if it could even be that much. Certainly not what you are describing.
You say the installation is pushing 35 years of age; I suppose it is possible that connections could be going bad, but I'd feel a whole lot better about that possibility if you had five different readings spread randomly across the 200A to 560A range, rather than four at the low end.
There is something very wrong here, and it is difficult to come up with anything that seems reasonable without a lot more information. No promise that any reasonable suggestion come from further information, but the information will certainly help.