Here's how the zero-level values work: when
$a is compared to
$b, if either value is below the
$zeroLevel value, which is
1E-14 by default, then if the difference between the two is less than
$zeroLevelTol, which is
1E-12 by default, then they are equal. That is, if
abs($a) < $zeroLevel || abs($b) < $zeroLevel, then the two are considered equal if
abs($a - $b) < $zeroLevelTol, and are not equal otherwise.
In the case discussed here, the correct value is given by
pi/6*500*cos(pi/2), which evaluates to
1.60305891143483e-14, and the test value is
500*cos(pi/2), which is
3.06161699786838e-14, neither of these is less than
$zeroLevel, so the usual relative tolerance rules are used, and they are not equal by that measure (since they would have to agree on their first 3 or 4 significant digits).
On the other hand, when testing against
100*cos(pi/2), which is
6.12323399573677e-15, this value is less than
$zeroLevel, and so the rules above come into play, and we must compare the difference, which is
-9.90735511861148e-15, to the
1E-14 in absolute value, and since it is less than the tolerance, these are considered equal.
When testing against the answer of
0 itself, since that is less than
$zeroLevel, the zero-level rules are used, and since the difference between
1.60305891143483e-14, and that is less than the
1E-12, these are also equal.
260*cos(pi/2), which is
1.59204083889156e-14, neither is smaller than
$zeroLevel, and so the usual relative tolerance is used, and this don't match
1.60305891143483e-14 to enough digits, so they aren't equal. On the other hand,
262*cos(pi/2) does match to enough digits, so that does match
So that is how it works, and why these examples do what they do.