I had to dig it deeper and I think the wrong text length becomes from html line breakers because the jericho html-parser for some reason replaces new line breakers with spaces or something...
As for now, I cannot say for sure which more tags does it replace to which characters but as for my case I just tried to do some walk-around using regular expression like this (see snippet)
html=html.replaceAll("<br>","");
Source source = new Source(html);
String text = source.getTextExtractor().toString();
... so now it really returns original text length as 42 :)
I hope the tip saves one day
Thank you all for help