El Niño and La Niña occur irregularly, which means that in a given decade or decades, there's no guarantee that the number, strength, and duration of El Niño and La Niña events will match exactly. A decade or two might have more La Niñas than El Niños, so that when you average the sea surface temperature patterns out over that time period, you will see the fingerprint of La Niña on the long-term average. But that is not the same thing as saying La Niña lasted the entire period.