Using 'for' to loop a loop

Okay I am still missing something. I think I follow what you are doing in your example. But it caused the program to crash so for now if I just stick with the less efficient and probably more confusing method.

I have changed my code a little.

ofSetColor(abs(curserx-300),abs(curserx-300),abs(cursery-300));
ofRect(0,0,600,600);
for ( b = 5; b < 610 ; b += 30){
    for ( a = 0; a < 100 ; a++){
        ax1[a] = 5 + a * 30;
        ay1[b] = a;
        ax2[a] = (abs(curserx)/30 - a);
        ofSetColor(abs(curserx-300),abs(cursery - 300),180);
        ofSetLineWidth(2);
        ofCircle(ax1[a],ay1[a],ax2[a]);
    }
}

The result from this code is the image below:
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Clik here to view.

I am more than confused what exactly I have done to now make the 6th circle offset down twice but nothing else...

What part of my code is causing this and can you explain why?