Assuming zero geometric error (e.g. Your Rad / Alt transmitter to the upper part of the "Target" aircraft below would be less than 2000 ft, any rounding down of the digital read-out (Rad / Alt digital read-outs are never rounded up), 2000 feet of Pressure Height delta converts to 1920 feet geometric difference on an ISA - 9.15 C day.
Such a non-standard atmosphere would be evident to you at F/L 310 and Mach 0.827 (as shown on your attached photo) as a SAT of -55.57 C and a TAT of -15.18 C.
Best Regards,
Old Smokey