Every scholar who reached to the degree of Ijtihad in which he properly deducts the evidence of Islamic rules from the main four sources (Quran, Hadeeth, Aql and Ijmaa') studies and understands the evidence in his own way and we can never inforce same understanding on all the Mujtahids.
Tattoo is allowed because we do not find any authentic evidence against it which can make it Haraam.
Wassalam.