There are three main reasons why a 'fully charged' cell does not stay at 4.2 V after charging. Firstly the cell has internal resistance, so any current going into it will raise the terminal voltage, while any current drawn from it will lower the voltage. Secondly, most chargers do not put in a full charge. Thirdly, voltage will gradually drop after charging as the charge 'settles' into the cell.
Here is the actual charging profile of a typical Li-ion cell:-
The voltage (red line) increases faster in the latter part of the 'constant current' phase, until it hits 4.2 V. Then the current (green line) must be reduced to prevent the voltage rising any further (the 'constant voltage' phase).
When current drops to ~10% of the 'constant current' rate, the charger shuts off (yellow line). At this point the battery is not quite fully charged (had the charger not cut off it could have pushed a few more mA into the cell). The voltage immediately drops slightly because there is no longer any current flowing through the cell's internal resistance.
Over the next hour the voltage drops slowly as charge 'settles' inside the cell. After a few hours the voltage should stabilize and only go down very slowly due to internal leakage.
How close the cell gets to full charge depends on the charger settings (charging current, cutoff %, peak voltage) and the internal resistance of the cell. 'High capacity' cells usually have higher resistance and so have to be charged at a lower current to avoid excessive voltage drop. As a li-ion cell ages its internal resistance increases, causing greater voltage drop and less charge put in before cut off - unless a lower charging current is set to compensate for the aging.
New cells typically have a 'resting' voltage after charging of ~ 4.16-4.18 V. A lower than expected resting voltage could indicate an old or faulty cell, excessive charging current, or a charger that is cutting off too early or not putting out a full 4.2 V.
