If I understand your question right, you are placing a coordinate system at the periphery of a circular path, with the y-axis always pointing tangentially. In this setup, you are comparing polar with Cartesian/rectangular coordinates.
The polar and rectangular coordinate (basis) vectors may appear to point in the same direction at first glance, but they are not of the same dimension.
One is
$$\text{Cartesian:}\quad\Big(\text{(tangential) acceleration}\;\;,\;\text{(perpendicular) acceleration}\Big)$$
while the other is
$$\text{Polar:}\quad\Big(\text{(radial) acceleration}\;\;,\;\text{turning angle}\Big)$$
In other words,
- metres-per-second-squared by metres-per-second-squared and
- metres-per-second-squared by degrees (or radians if you will).
The angular coordinate in a polar coordinate set can never equal a Cartesian (rectangular) coordinate, simply due to its different dimension (different unit).
Thinking correctly of the angular coordinate as a number-of-degrees also gives you the impression of a curved axis and basis vector rather than a straight vector. It doesn't say "how much" in a straight direction, but "how much" around. Such curved basis vector is clearly not a tangent to the curve - rather, it is (it defines) the curve.
Now, since the angular coordinate in one system can't equal the tangential coordinate of another system, in a polar coordinate system the radial coordinate is the only one left to carry the entire "size"/magnitude of the acceleration.
In Cartesian (rectangular) coordinates we are used to the magnitude of a vector being shared between both coordinates, and we find the magnitude as a mix (through Pythagoras' relation). So clearly, none of the coordinates in a Cartesian coordinate system equals the radial coordinate in a polar system - although the dimension (the unit) fits, they carry different information. They are not the same thing.
Conclusion: There is no overlap between the coordinates of polar and rectangular systems. No coordinate in one equals that of the other. None of them fit to the other.
The way to think of these two system is thus
- intuitively go-a-bit-out-and-a-bit-up for Cartesian coordinates while
- less intuitively go-the-full-amount-out-and-turn for polar coordinates,
and you can only convert from one to the other through the well-known trigonometric sine, cosine and tangens relations in a right-angled triangle.