[adgjlsfhk1]

It's not different rounding modes, but different floating point formats. Google is doing something very weird that is also base 10 related. Not sure about TCL.

[tzot]

TCL does for quite some years now use real integers (unbounded size) and floats (float8) whenever possible. By the way, the article does not specify versions of languages, so:

    $ tclsh
    % puts $tcl_version
    8.6
    % expr "9999999999999999.0-9999999999999998.0"
    2.0