Interesting discovery, this only happens with some numbers but not others. I am even more confused:
>> -1*1e-9
ans =
-1.000000000000000e-09
>> -2*1e-9
ans =
-2.000000000000000e-09
>> -3*1e-9
ans =
-3.000000000000000e-09
>> -4*1e-9
ans =
-4.000000000000000e-09
>> -5*1e-9
ans =
-5.000000000000000e-09
>> -6*1e-9
ans =
-6.000000000000001e-09
>> -7*1e-9
ans =
-7.000000000000001e-09
>> -8*1e-9
ans =
-8.000000000000000e-09
>> -9*1e-9
ans =
-9.000000000000001e-09