I wouldn't worry too much about weather forecasts. I've just recently returned from Cyprus and out of curiosity whilst I was out there, I occasionally checked what the UK Met Office and the BBC weather reports were for where I was staying. More often than not, they were wrong and sometimes quite badly wrong. On more than one day, both the Met Office and the BBC claimed that it was raining when it was actually cloudless and “scorchio” outside. More often, they would claim that it was cloudy and about 26 degrees when in reality, one or two wispy clouds might be seen if you strained your eyes and according to our car's thermometer it was 32 or 33 degrees. The heat was palpable and pleasantly consistent.
Islands of any significant size (and Lanzarote is not a particularly small island) are subject to huge variations in local weather conditions. Most temperature readings on small to medium size islands are taken close to or at the largest town or city. Extrapolating the weather conditions at one precise location and arbitrarily applying that to the whole island is common practice in weather forecasting and can lead to quite misleading information.
Have a nice holiday!