The description of the QVariant::toString function reads:

Returns the variant as a QString if the variant has userType() QMetaType::QString, QMetaType::Bool, QMetaType::QByteArray, QMetaType::QChar, QMetaType::QDate, QMetaType::QDateTime, QMetaType::Double, QMetaType::Int, QMetaType::LongLong, QMetaType::QStringList, QMetaType::QTime, QMetaType::UInt, or QMetaType::ULongLong; otherwise returns an empty string.

However, there are several other values of QMetaType that will generate a non-empty string, namely: QMetaType::Float, QMetaType::Short, QMetaType::UShort, QMetaType::Long, QMetaType::ULong, QMetaType::SChar, QMetaType::UChar, etc...

I can only assume that this intended behaviour (why would Double be printed and not Float??), so I believe that the documentation is incorrect.