if statement condition with calculation inside problem

I am quite new to c# and unity.

I have a if else statement which confuses me.

 public float onTime;
 public float offTime;
 public float flashIndex = 0;
 public float repeatTime = 0.1f;
 
 IEnumerator ObjcBlinking()
     {
         yield return new WaitForSeconds(repeatTime);
 
         float time = onTime + offTime;
 
         if (flashIndex <= onTime)
         {
             rend.enabled = true;
         }
         else if (flashIndex <= (onTime + offTime))
         {
             rend.enabled = false;
         }
         else
         {
             flashIndex = 0;
         }
 
         flashIndex += repeatTime;
 
         StartCoroutine(ObjcBlinking());
 
     }

The else if (flashIndex <= (onTime + offTime))doesn't work correctly. But if I changed it into else if (flashIndex <= (float)(onTime + offTime))it works fine.

Just wonder why I need a type transform for this case to make it work properly?