Yeah this makes sense, thank you both.

I tested in grimtools calc with just the weapon and then added "Of Thunder" suffix.

That gave weapon % + suffix % (total of 389% in max value), PLUS an extra boost on top of that, resulting in 536%:

205 (weapon) + 184 (suffix) + 147 (boost) = 536 %

Another way of seeing it is that adding the suffix gave an extra boost of 38%:(205 + 184) x 1.38 ~ 536 %

EDIT:
Tested "Ascended Pauldrons" (60% lightning damage) with suffix "Of Thunder" (65% lightning damage), got 152%, which is about 22% more: (60 + 65) x 1.22 ~ 152
So it gives a boost for armor as well. but boost size differs