Strange (interpolated maybe) values from openlayers canvas - webgl

Question

I create an openlayers map application for creating histogram from COG layer. I using WebGLTile layer and DataTile source. In a map I have only 2 colors but gl.readPixels returns 12 color values.
map screenshot
part of code:
map.on("click", (evt) => {
var buffer = {};
const gl = webgl.getRenderer().helper.getGL();
const pixelData = new Uint8Array(gl.drawingBufferWidth * gl.drawingBufferHeight * 4);
gl.readPixels(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight, gl.RGBA, gl.UNSIGNED_BYTE, pixelData);
let r, g, b, c = 0;
for (let i = 0; i < pixelData.length; i+=4) {
r = pixelData[i];
g = pixelData[i+1];
b = pixelData[i+2];
let item;
item = [r, g, b].reduce((acc, cur) => {
if (!acc.hasOwnProperty(cur)) {
acc[cur] = {};
}
return acc[cur];
}, buffer);
item.count = (item.count || 0) + 1;
}
});
In final buffer I need only
{
"165": {
"1": {
"38": {
"count": 1181613
}
},
},
"255": {
"255": {
"191": {
"count": 351
}
}
}
}
but I get 12 colors between these two intervals.
{
"165": {
"1": {
"38": {
"count": 1181613
}
},
"2": {
"39": {
"count": 130473
}
}
},
"168": {
"10": {
"43": {
"count": 130413
}
}
},
"169": {
"11": {
"44": {
"count": 15151
}
}
},
"174": {
"27": {
"54": {
"count": 130481
}
},
"28": {
"54": {
"count": 14792
}
}
},
"177": {
"36": {
"59": {
"count": 14460
}
}
},
"178": {
"37": {
"59": {
"count": 1631
}
}
},
"242": {
"219": {
"170": {
"count": 130366
}
}
},
"243": {
"220": {
"170": {
"count": 14487
}
}
},
"246": {
"228": {
"175": {
"count": 14884
}
},
"229": {
"175": {
"count": 1574
}
}
},
"251": {
"245": {
"185": {
"count": 15250
}
}
},
"252": {
"246": {
"186": {
"count": 1586
}
}
},
"255": {
"254": {
"190": {
"count": 1528
}
},
"255": {
"191": {
"count": 351
}
}
}
}
I have no idea why is this happenning and I spent days searching for solution.
I found some example, not with openlayers, and this approach was fine. returns only two values from double-color square.
It's some kind of interpolation on the openlayers side?
Thnaks for any help!