Why does the arb calculator produce different results?

[mtx]

for i.e. for a ML bet, Book A shows Team 1 -206 Team 2 +204 and Book B shows Team 1 -240 Team 2 +200. If I wanted to arb for Team 1 -206 and Team 2 +200, what would be the correct method in determining my negative EV?

The Arb Calculator at http://arbcentral.com/tools/arbcalc.html shows that if I put a -206 Line at Book A with an Amount of $2000, and a 200 Line at Book B with an Amount of $990.29, I would win $970.87 or lose $2000 at Book A and I would win $1980.58 or lose $990.29 at Book B with an EV of -$19.42. However, if I reverse the lines and put a 200 Line at Book A with the same Amount of $2000, and a -206 line at Book B with an Amount of $4039.22, I would win $4000 or lose $2000 at Book A and I would win $1960.78 or lose $4039.22 at Book B with an EV of -$39.22.

So just by switching the order of the lines (with the same bet amount), the EV doubles.

Is there an clear explanation for this?

Should I always calculate both sides and take the arb with the best EV? I've been always putting the bonus sportsbook as Book A, but it seems like putting it as Book B may yield a better EV sometimes...