good question Marat - in English with numbers "a " means 1 - eg a thousand = 1000 a hundred = 100 --- the article is describing the number not the items that there are a number of , a bit tricky , I hope that helps :-)
We do the same in English, eg, Give me a hundred dollars. It's the same as saying, give me one hundred dollars. Not saying it's right, but certainly not a foreign concept.