So according to the conversion table I have a 46 gallon tank when it takes me 11 5 gallon buckets to fill it still trying to figure this out. I know this because I made my gf count... my tanks a 48"x12 and like 1/4Wx 18"H
It's probably because you aren't actually getting five gallons of water each time. To be a full five gallons, the bucket would have to be full completely level with the top. This would make it practically impossible to move without spilling.
Many tank companies rate their tank capacities different. For example, I have a 150g that on the conversion table comes out to 147g. Another thing to think about is factoring in the glass thickness when using the conversion table.
Yes my my '220' is not 220 when you compare the inner dimension to the outer, I loss 1 inch front to back and one side to side due to the acrylic. My 500 loses 3" front to back and another 3 side to side.
Plus overflows and ect
I also think that a 5 gallon bucket is more then 5 gallons there is normally a small head room left.