I'm not a scholar regarding the Psalms so I'm throwing this out there if a change needs to be updated in the Psalms Explorer.
All places I've read mention David being the author of 73 of the 150 psalms, yet Psalms Explorer states that David is the author of 74 psalms. Also, when I add the authors from Psalms Explorer with the rest of the authors (excluding the LXX authors) there is a total of 151 psalms, which leads me to think the mistake is with Psalms Explorer.
There is one other minor discrepancy: when I click on David (author of 74 psalms), then break it down to structure...the math tells me he is the author of 78 psalms. Either the math is wrong or a psalm has multiple structures applied to it?