When a battery is being charged, the electrolytes near the electrodes (both positive and negative) receive the charge first and then it disperses throughout the battery. The latter is a slow process.
So if charging at a higher rate, only the part near the electrodes get charged up and shows full charge voltage of 4.2V but the parts farther from the electrodes haven’t received much charge yet.
Actually the rate is slowed significantly once you reach 80 percent(provided you are using the charger your phone came with and then again when it reaches 95 percent but actually it is slowing down once each percentage is crossed but only significantly at about 80 or 70 percent depending on if you are using the original charger.
Once the battery reaches closer to full charge if the charging current is not reduced it leads to release of gases which expand the battery and can even cause it to explode your phone and kill you or disfigure you.
To prevent this your phone is charged in a 3 stage or 2 stage charging method where from 15 percent to 80 percent a lot of current is allowed to enter your phone
Once it reaches 80 percent it reduces charging rate and thus tries to stop any damage due to expansion of battery by gassing and this is further reduced once it reaches about 90 percent where the charging current is further reduced and after this the current drops slowly and charging from 99 percent to 100 percen takes the most time.