SQL & PL/SQL :: IDs Decimal Values Are Truncating

If I perform a Query as below

SELECT SUM(total_amt) FROM Amont_dtls WHERE id=10;

It's giving me the value as

2.08166817117217E-19

The values in total_amount column for ID=10 is 0.01 and -0.01..Actually it should return 0.If I use the query like

SELECT SUM(0.01-0.01) FROm Amont_dtls WHERE id=10;

Then I am getting 0.Why it's a variation.And also the data type of the column is NUMBER.If I use

SELECT TRUNC(SUM(total_amt)) FROm Amont_dtls WHERE id=10;

Then I am getting 0.But some other ID's decimal values are truncating.

Ex: If i Use TRUNC
Instead of 28781947.48 it's showing 28781947

Instead of 590874.5 it's showing 590874