Hola Kevin

When we use time markers using "este..., esta...", e.g. esta tarde, este verano, estos últimos días..., etc we feel as if we are still inside that timeframe, and that makes us use the perfect tense instead of the preterite, in Peninsular Spanish. This is why we use it here in this exercise using "este verano". 

Here is a Kwiziq lesson that will help you with this usage. 

I hope it helps.

Saludos

Inma