The registrations are "recyclable", so when a certain registration is released form a plane that is either removed from service or re-registered to a different country, the previous registration can be used on another plane. This is not optimal if done immediately as it does create confusion sometimes as databases may "lag".
On a further note, sometimes Flightradar24 and other such services simply get these things messed up, showing wrong registration, type and/or picture of plane. This particular case is about re-registration.
You can use planespotters.net to check registration history or aircraft, here is the case for OE-LNL: planespotters.net - OE-LNL. As a perfect case example of the aforementioned database faults, this service has the very first entry wrong (as I'm writing this, database may become corrected later), but both registration entries mentioned in the question are present in this database:
OE-LNL - Boeing 757-200 (current registration at the moment)
Click to embiggen
OE-LNL - Boeing 737-600 (removed from registry July 2012)