The Converter works on a "least common denominator" system. Galleons, Sickles and Knuts are first converted to Knuts. The total Knuts are then compared to cents or pence depending on the conversion requested.
The exchange rate used in the converter is based on the price of the book "Quidditch Through The Ages". American copies of the book are priced at $3.99 and UK copies are £2.50. They also both state that they cost 14 Sickles and 3 Knuts (409 Knuts). Since Knuts do not divide equally into muggle money, inaccuracies in calculations of a Knut or two may occur due to rounding.