B737 vnav function
 

Hi, guys

I am a newly type-rated pilot on B737 and have some questions about VNAV funtion.

I was on a ILS approach, and early leveled on a certain altitude. Let's say that it was 4,000ft.
I was directed to an waypoint by atc, so I engaged LNAV. And I was in F/5 config, surely under vertical path(I could convinced by DME distance and VTK, too), so I engaged VNAV to follow vertical path.

Then suddely my plane started to fly up as if it tries to catch up VTK so I pushed ALT HOLD, engaged APP mode and finished the flight.

My question is, why did it tried climb up to catch VTK? I was under vertical track and LNAV was engaged in advance. So it should maIntain the altitude and catch up th vertical path at 4,000, I think. Don't you agree? What did I do wrong?