Others have stated that the overall rank is just calculated independently based on all ranked games.
These above two types of discrepancies can be explained if players have had a general trend of improving or declining (or at least just a general winning or losing streak) over a period of time, while also playing a mix of all three game types. The overall rating will have moved much faster since it has roughly three times as many adjustments as the categorical ratings. In these cases, its just that overall rating is moving quicker toward the more accurate value, and all four should eventually stabilize as they converge toward the player’s true rating. However, small discrepancies are always still possible from winning/losing streaks.
Calculating the overall ratings as an average would create other issues. If a player simply never played games of one or more of the types, that unchanging component of the average would suppress any changes in their overall rating.