T antijoin itself returns still T?

[unknown1337]

I'm trying to implement a not operation using antijoin. But i'm having an issue, simplified:
in a browserconsole:
(T.antijoin(T)) returns
ColumnTable {_params: {…}, _names: Array(6), _data: {…}, _total: 1093, _nrows: 146, …}

How can this result deviate from 0 rows as all rows in table T are in table T?

Thanks!