Why there is a discrepancy in data on migrants from Croatia to European countries, depending on the source we consult, and specifically in the period since Croatia's accession to the EU, is the main question addressed in this paper. In that context, the paper focused on voluntary, international or external migration, primarily for economic reasons, from Croatia to six European countries. The differences between the numbers of reported Croatian immigrants in the national statistics of the six European countries - Austria, Italy, Germany, Ireland, Sweden and Norway - and the numbers of reported immigrants in the Croatian national statistics indicate some deficiencies in the methodology used for migration statistics. The analysis showed that, according to the latest available data, Croatia does not record nearly half of its displaced citizens in comparison to the records maintained in the observed countries. In order to reveal the background of such disproportion, further analysis compared data sources for migration statistics, variables used in migration measurements, and regulations and circumstances of registration / deregistration of residence. The analysis revealed several key elements that need to be introduced or harmonized at national and European level: establish and keep up to date administrative registers at both levels, introduce a variable country of departure at European level, and design and implement measures of verification of actual residence and promptly report them at national level.