I found out that that from version 1.8 a rule was introduced "Normal trading yields 3-6 experience points, trading and making a villager willing yields 8-11 experience points" as it is described here https://minecraft.fandom.com/wiki/Java_Edition_14w02a. I have tested it rule is working til version 1.13.2 (including). From 1.14 and later versions doesn't follow rule and every trades despite villiger willingness gives 3-6 xp points. Was this rule removed on purpose? As well I have noticed that villager data NBT tag- "Willing" was removed since 1.14.