bug or not in GPP percentage?

I had built in my city the Oracle, so it should give me a +2 to GPP priest.
In the same time I added a scientist specialist which gave me +3 to scientist GPP.
Anyway while i got +5 points to GPP i had a 50% chance to get both a priest and a scientist.
Shouldn't be 60%/40% in favour of scientist?
If it functions as planned having for example 2 wonder which both give 2 priests and 2 scientists as specialist will i get the same 50% chance for scientist and priest?

 

You are wrong. "X% chance to get X GP" is defined by the average number of "source" of GPP. A W-wonder is 1 source, a N-wonder is 1 source and 1 specialist is a source. The number of GPP produced by each source doesn't matter to calculate the "X% chance to get X GP".

There are some articles explaining this

 

Not really... it's cumulative, instead of instantaneous... the points have sources identified.

Consider it this way... discounting wonders for the moment, if you have a pile of Priests in a city, and let them do their thing until the turn before a GP would appear, and then suddenly switch them all into Engineers... you'll probably get a Great Prophet (unless you're really lucky). Extending that to your scenario, if you have the Oracle built a long time ago, and then you suddenly hire a Scientist, you have a whole heap of Oracular power, and not necessarily a lot of scientific might. Over time, this will slowly equalize (your moment), and then shift towards 60-40. After your next GP, it will equal 60-40 in the future, unless you do something else to change things.