It is not clear what ht 83% is., Maybe a typo.
Seems quite clear to me. From what you quoted: "The volume of spam in mail traffic increased slightly compared to May and averaged 83.3%"
It means that in May, spam email traffic was to all email traffic as 83.3 is to 100. Expressing the relation of a part to the whole in this way is called giving it as a "percentage", from the Latin
per centum. It is a convenient method of expressing a fraction in a quickly (I thought) understandable way. The formula is (part/whole) x 100. That symbol like a slash with two little circles is called a "percent" sign or symbol.
If s is spam traffic and m is all mail traffic (s/m) times 100 = 83.3.
The figure seems to be in the right ballpark:
For some time now, spam has been accounting for upwards of 90 percent of all email messages.
(Threat Weekly, Kapersky Feb 2010)
With a share of 96.4 percent of the total email volume, however, spam remains at record levels. The top spam originators in July 2010 were the US, followed by Brazil and India. Germany dropped to seventh place.
(Web Host Industry Review, August 2010)
Spam – 72.9% in June (a decrease of 2.9 percentage points since May 2011
(Symantec Intelligence Report: June 2011)
...in fact it seems to have gone down a bit over the last year or so, but I guess concluding that depends on using comparable measurements.