I think he's saying that he's been using 4.8 and installing on his customer's computers, then he tried v.5 on his to see if it was stable before he started putting it on his customers' machines. He feels that it slowed his computer down, but was thinking of blaming it on a clogged CPU cooler, and hasn't had time to clean it yet. He then tried several others to test the commandline scanners, as he believed that the machine did in fact have a virus, and Avast! v.5 boot-time scanner caught some. He feels that v.4.8 should have caught those viruses before.
Correct me if I'm wrong on the interpretation. I'm not an expert, but I am familiar enough with Eastern grammar to be able to pick out most meanings in cases like this.
My personal answer is simply that no antivirus can catch everything, but v.5 has made significant improvements to detection. I think if you had run the 4.8 boot-time scanner it would have caught them as well. Avast! is the best by far, but nothing is perfect, and the evil coders are working overtime to get around the AV people. When stuff slips through, just update and boot scan. That's why Avast! excels at detection AND removal.