To get the most accurate average return you could take each separate investment, apply the compounded interest formula from the time of investment to now, and sum them. Perhaps tedious but doable using a spreadsheet. Use one cell to hold the interest rate, then use Excel's Tools>Goal Seek to adjust the interest rate so the sum equals your current account value.

Somewhere in between, you could take some weighted average of the time invested. E.g., if you have been putting in about the same amount every year, (37863.74/19900)^(1/(14.75/2)) - 1 = 9.11%. This is not "correct" but it would be "closer to correct" - and perhaps close enough for your purposes. It's also closer to what the fund itself claims, and there is at least a chance they are correct.

It appears to have been a good investment.

Still curious, however, about those "Distributions paid in". One could say "19900 + 35668 = 55568 paid in, but now worth only 37864"...?