How many bytes is a gigabyte (GB)?

The two tools are converting two different units. 1 GB = 10^9 bytes while 1 GiB = 2^30 bytes.

Try using google converter with GiB instead of GB and the mystery will be solved.

The following will help you understand the conversion a little better.

Factor  Name    Symbol  Origin  Derivation   Decimal
 2^10   kibi    Ki  kilobinary: (2^10)^1    kilo: (10^3)^1
 2^20   mebi    Mi  megabinary: (2^10)^2    mega: (10^3)^2
 2^30   gibi    Gi  gigabinary: (2^10)^3    giga: (10^3)^3
 2^40   tebi    Ti  terabinary: (2^10)^4    tera: (10^3)^4
 2^50   pebi    Pi  petabinary: (2^10)^5    peta: (10^3)^5
 2^60   exbi    Ei  exabinary:  (2^10)^6    exa:  (10^3)^6

Note that the new prefixes for binary multiples are not part of the International System of Units (SI). However, for ease of understanding and recall, they were derived from the SI prefixes for positive powers of ten. As shown in the table, the name of each new prefix is derived from the name of the corresponding SI prefix by retaining the first two letters of the SI prefix and adding the letters bi.

There's still a lot of confusion on the usage of GB and GiB in fact very often GB is used when GiB should or was intended to be. Think about the hard drives world: Your operating system assumes that 1 MB equals 1 048 576 bytes i.e. 1MiB. Drive manufacturers consider (correctly) 1 MB as equal to 1 000 000 bytes. Thus if the drive is advertised as 6.4 GB (6 400 000 000 bytes) the operating system sees it as approximately 6.1 GB 6 400 000 000/1 048 576 000 = ~6.1 GiB

Take a look at this for more info on prefixes for binary units and this on metric prefixes.

This is just a confusion of units. There are actually two prefixes G for 10⁹ and Gi for 2³⁰. Bytes should usually be measured with the second, so the correct writing would be GiB.