Why I have monocytes, absolute elevated if everything else in CBC (includes diff/plc) test is normal?

Probably no reason. The reference range is set so that 5% of healthy folks will test high. I'm going to assume there are no abnormal forms, that you do not have anything to suggest typhoid fever or advancing tuberculosis, and that this is an anomaly found on routine lab work. If so, stop worrying about it.
Not of concern. A thousand different reasons most of them benign or of no concern when the rest of the CBC is normal. It is nothing to even be concerned about. The CBC is to be taken as a whole in general.
Multiple reasons. An increased absolute monocyte count may be associated with numerous conditions including chronic infections, inflammatory states, some autoimmune diseases, and even certain types of leukemia, but a more specific comment cannot be provided based on the information you offered. It is also important how high is the monocyte count. If only slightly increased, it may mean nothing.