A student with a solid mathematics background from the Turkish curriculum often finds the start of AP Calculus surprisingly easy: derivatives and basic integration are already familiar. The comfort ends near the BC syllabus, with a specific thought: I have never seen this topic.
Why a gap exists at all
AP Calculus AB and BC have an official scope set by the College Board, designed around the American high school curriculum. The Turkish curriculum also covers the foundations of derivatives and integrals thoroughly, so at AB level the two overlap substantially.
BC adds units on top of AB, and those additional units do not appear in the Turkish curriculum. The problem is not difficulty, it is scope: the student is strong in what they already know and starts from zero in what they have never seen.
The three topics that are missing
Differential equations, particularly separable equations and slope fields, are not covered in the Turkish curriculum.
Parametric, polar and vector calculus requires an entirely new notation and way of thinking: derivatives and integrals of parametric equations, area in polar coordinates, vector valued functions.
Infinite sequences and series, with convergence tests, Taylor and Maclaurin series and error bounds, is one of the densest and most heavily examined parts of BC, and it does not appear in the Turkish curriculum at all.
These three areas make up a significant part of the BC exam. A student who sits it unprepared performs well on the AB material and loses serious marks in these sections.
Why starting over is the wrong fix
Some students who notice the gap decide, to be safe, to work through AP Calculus from the definition of the derivative onwards. This wastes time and lowers motivation, because the student sits through material they already know.
The efficient route is to work only on what is missing: leave derivatives and basic integration alone and go directly to differential equations, parametric and polar and vector calculus, and series.
Which student needs which
A student who knows the Turkish curriculum well and needs only the additional BC topics is served by a targeted syllabus gap module.
A student from outside the Turkish curriculum, from a foreign school or a different system, or with a weaker mathematics foundation, is better served by a combined course that covers the full official scope without assuming anything.
The choice between AB and BC
BC is generally the stronger signal for engineering, science and economics. Jumping to BC without a solid AB foundation is risky, because the additional units assume the earlier ones are secure.