Why do the number of playable arbs differ so much?
Generally, the closer to an event it is, the more arbs appear. This is due to the fact that bookmakers adjust their odds more frequently closer to a match (depending on lots of factors such as the weather, injuries, team/player form, rumours and so on). Some bookmakers also adjust odds in order to attract more customers closer to an event (to even out their risk, they always want about the same monetary risk on all outcomes).
The number of arbs available therefore depends on season (what events are running), day of the week and even time of the day. Most often there are more arbs at the end of the week (Thursday-Sunday) because more matches are played at the weekend.