One way that this "0.5" could make sense, is:
I assume the guild used in Sirrians example has 11 members.
That means that if the Paragon shall be able to get 100% bonus based on the other members, each of the 10 other members will provide for 10% each.
Since Sirrian in this example is a Champion, he can hence only give bonus to one player over him, and that means 10% of +5 Magic = 0.5 Magic.
I do not know if this is the correct way to add up the numbers, but if it is, then if we look at a Herald with also +5 Magic (assuming there are 2 Champions in the guild):
The Herald would provide 0.5 to the Paragon and 0.375 to each of the two Champions, a total of 1.25. So for a Herald the text would read "+5 Magic, +1.25 for higher ranked Guildmates".
Also back to the original example with a Champion. If Sirrians guild had 21 members in stead of 11, then each of the other 20 players would provide 5% to the Paragon, and Magic=5 would only provide "+0.25 to higher ranked Guildmates".
I was wrong in the line-through text over. In a guild with 11 members with 1 Paragon and 2 Champions, there are only 8 players to provide for the maximum 75% Bonus a Champion can get. That means each player below the champion provides for 75% / 8 = 9.375 %.
Hence the Herald would provide 0.5 to the Paragon and 0.47 to each of the two Champions, a total of 1.44. So for a Herald the text would read "+5 Magic, +1.44 for higher ranked Guildmates".
EDIT 2: In the numbers over, I also assume that for instance a Paragon can get max 100% bonus independent of the total guild size.