There are two sorts of limits that you are mixing up.
The first limit is clearly explained in the holes theory post on my website. It is a limit placed on the country during the draw process. Too many entries come from certain countries and so during the draw when they reach enough winners from that country they simply knock out the remaining winners from that country. The impact of that is to create the holes and reduction in density that the graph above clearly shows. The higher the number of entries, the sooner the country meets the draw limit. There are 3 countries limited for certain (Ghana, Egypt and Ethiopia), but there is at least one more country limited (either DRC, Liberia or Cameroon). Of this second group you could argue that all three are in fact limited (based on the fact that none of them exceed the 5000 selectees limit) but it appears a to be a fairly significant drop in density at about 52 - 54k - which might be a 4th country or it could even be two of the countries maxing out at about the same time.
The other sort of limit is applied during the visa issuing process. The law says no one country can exceed more than 7% of the available visas (50k) in any year. So there is a limit in place of 3500 visas to any one country. By using NACARA visas that limit can be 7% of 55000 (3850). So we could see a country max out that 7% limit. However, to do that a country needs a lot of selectees and a high success rate. Only two countries in the world have a chance of doing that - Nepal and Egypt. To avoid calling people to interview when a country is close to the 7% limit, KCC can leave a VB limit in place at the end of the year (as happened in DV2014 with Egypt at 32250 and Nepal at 9500).
The other monthly limits in the VB that we see at the moment for Egypt and Ethiopia is to control the pace of their workload - so that they don't have to process all the work in the first 3 or 4 months of the year. Ghana embassy doesn't seem to need that control of pace, possibly because Ghanaian selectees have a high refusal rate (a refusal is less work that a visa issued), or possibly because the agents have created winners to blackmail and many of those cases don't show up for the interviews - thus reducing the workload.