The barrels are the same
The aft one is a vertical loading 155mm gun proposed in the 1980s for fire support tasks. It would have fit in roughly the same space as the 5"/54 Mk 42 (and was at one point proposed for a Knox class modification). The forward gun is a 5"/62 Mk 54 Mod 4.
As for the barrels being the same size, it's an artifact of the fact that a 5"/62 is 310 inches long while a 6.1"/55 is 335 inches long (only 4 pixels difference), and the fact that the barrel widths are sufficiently similar that they both get rounded to three pixels wide.