Let's say the shower has dimensions between 3 and 5 feet in one or more directions, and that it is typically made from reflective tiles or glass. That puts the eigentones, and their first harmonics into the 100 - 500 Hz range of the voice, that then is going to seem a lot richer than normal.
Perhaps, dare we say, another factor might be that the broadband sound of the water masks any imperfections in the voice!