computer/<name>/systemInfo page gather values for the table of environment variables from EnvVarsSlaveInfo class which uses Computer.getEnvVars() resp. Computer.getEnvironment() for obtaining an EnvVars structure.
The EnvVars class stores values as a TreeMap with the case-insensitive comparator. When the keys are different, but the difference is only in character cases, it stores all such keys in one key. It is un-deterministic which environment variable name you will see, it depends on the order returned by System.getenv() (the first occurrence is used as the key for map) - it's a natural behavior of TreeMap.
In case you have more keys with a difference in the case character(s) only you don't have a real view what is exactly the running environment.
EnvVars is used in many cases in the core hence I don't want to break a backward-compatibility so I'd propose a fix that don't use EnvVars for display this output, but obtain a full real environment variable collection from the agent at the time of page generation.