Torture tests: what do they mean?

[jebem]

This site calls my attention to how different calculators are concerning the used algorithms to get a result for a specific problem.

For example, the tan(355/226) has an ideal result of -7497258.185...
It gives me the following results (in Radian mode) for these models:

HP-Prime: -7497089.06508
HP-50G: -7497089.06508
Casio FX-991DE PLUS: -7497258.44
WP-34S: -7497258.194

Clearly the HP models algorithms deviates somehow from the expected result, contrary to what the Walter&Pauli algorithms which are giving fantastic close results.
My new acquisition, the Casio FX-991DE PLUS, is a nice surprise with a close second in this torture test, probably because it uses 15 digits internally like many others Casio calculators.