Hola Vasilis

The reason why the tense in different in English is because we use the time frame "esta mañana" differently in European Spanish and English. In Spanish when talking about actions happening "esta mañana" (this morning) we see ourselves still inside that frame and we use the perfect tense (e.g. he visto), however, in English the preterite tense is used generally when using the time frame "this morning", therefore the English translation there is "saw" (not have seen).

This lesson talks about this. Have a look here.

I hope this clarifies it.

Saludos

Inma