Hi! Thanks for contacting us with your question! The reason, that the datalabel isn't centered is probably because you are defining font-size in other place, in datalabels object. So the font-size is the option, that makes the div, in which the datalabel is placed, assume the size of the text based on this option. Check this demo, where I changed the font-size in dataLabel options definition to be equal to the size, where you define it only for the first bar.
https://jsfiddle.net/BlackLabel/zxf5brqc/
I think, that it is better to manage the datalabels configuration through the options object.
In case of any further questions feel free to contact us again!
Kind regards,