The exact same transmitter and antenna, radiating the exact same real power — yet reported as two different numbers, 2.15 dB apart, because they're measured against two different reference antennas.
EIRP and ERP both try to answer the same practical question: how much power does this transmitter effectively radiate toward a receiver in a given direction? It's tempting to treat them as interchangeable — two names for the same thing. They aren't. Each one compares the real antenna to a different theoretical "reference antenna," and those reference antennas don't radiate equally well. That single difference in convention — not any difference in the real transmitter, real antenna, or real radiated power — is what makes EIRP and ERP genuinely different numbers for the identical physical setup.