Different enchant cost for same enchant

I have 2 pickaxe lets call them pickaxe1 and pickaxe2

On pickaxe1 was Efficiency IV and Fortune III - durability was ~25%

On pickaxe2 was Efficiency IV and Fortune III - durability was well almost 0

I enchanted pickaxe1 in anvil with Unbreaking III book.  It cost me 6 lvl. I had 34 and went to 27. Which was strange because I had to run back to farm cuz it cost 34levels to enchant Unbreaking III last time I looked. Then I put pickaxe2 into anvil with another Unbreaking III book and it showed me cost 33 levels.