The problem that I have detect it's that the avergage value that the dashboard it's calculating is the average of the whole datatable values without taking into account the relationships between tables. For example:
For this ID_LOTE:20170315232140R-245 in LOTES_TABLE which Codigo-Version R-245-150.0 (the filed which relate both tables)
The unique value related of peso metro in Spec table is 15,24 for the Codigo-Version R-245-150.0
So I really don't understand why it's displaying the average of all peso metro values in spec table, which is 10,38.
The average measure here will not consider that if the related value in another table is blank, it will do the average calculation for all values. If you want to only show the average for those values whose related value is not blank, you need to create a measure with some conditions.