That's one reason. But the main reason is whether visas are available (or expected to be available) to the selectees from that country or not. If the country-specific 7% limit has reached or is very close, there's no point in telling the selectees from that country that they are current.
Let's say a country's case numbers are stretched from 0 to 20,000. Let's assume at case number 10,000 that country uses up (or is expected to use up) all their 7% visas. The regional quota may progress beyond 10,000 or 20,000 or even 80,000. But you can't tell a selectee from that country with a 20,000 case number that they are current, because there is no visa for them. So they set a country-specific cut-off at 10,000 and keep progressing the regional cut-off.
Because it happened in the last month. If they expected it to happen earlier, they would impose a country-specific cut-off even if the whole region was current.
I think an important factor to look at is the stretch of case numbers from Iran. This year we know it goes as high as 8K. But what was the range last year? I'm not surprised if it was higher than 8K. If you have more participants in the drawing (which I believe Iran did this year) you will reach the country limit of 7% earlier in the drawing. So although Iran has almost an identical number of selectees this year compared to last year, they may be concentrated in lower numbers this year. Hence, a higher chance they may use up their 7% visas earlier in the year and a need for country-specific cut-off.
By the way, I keep saying "they" when talking about Iran, but I myself am from Iran