[Esorp]

The trend for electronegativity, is increasing when going up a row or across a period, but there are some exceptions which I'm confused about.

Tungsten is 2.36, directly to the right of it is Rhenium. According to the trend, one would predict the electronegativity for Rhenium to be higher, but it is much lower at 1.9. Gold is also an example of this at 2.54, Mercury next to it is 2.0.

So what causes this?

[AWK]

This is only trend, not smooth mathematical function

[Esorp]

Of course, but something is interfering with the trend. In the two examples I gave (Gold and Tungsten),  something is different about these elements to give higher electronegativity.