Ok, maybe mixing up decimal and percentage formatting is bad form on my part. It ought to be 'multiply by 1.45 and multiply by 0.75', or 'multiply by 145% and multiply by 75%', as opposed to a mix up in the same sentence.

Anyway, Chivalry and Beelzebub's attack buff. Chivalry is +10% to assault. Beelzebub's buff is +15% to assault. These two are also in the same 'frame' ('frame' is a google translate-ism of the jp wiki. There's no official in-game term, so we run with this), which means they do not stack with each other. Just a FYI.
+10% is a small enough increase such that you can possibly roll high with a buff-less attack, then roll low with Chivalry on, and the two numbers can look like they're basically the same. As for the +15%, the numbers shouldn't be that close to each other just yet.

Why I do say 'just yet'? A +X% attack buff doesn't necessarily mean +X% to damage. All the plain old +attack buffs you see add to assault/character. All the (X element) assault skills on weapons add to assault/character. Eidolon passive effects that say (X element) characters' Atk up add to assault/character. Key word: add. They're all lumped together into one factor/term in the damage formula.
If you only have 0% assault (like when you're just starting), then a buff that adds +15% to assault would indeed raise your damage by 15% compared to before the buff. But what if you already have some, like via an eidolon passive giving... +40% for example. Then that +15% atk buff would raise your assault/character value from +40% to +55%. The damage comparison would then be 155%/140% ~= 110%. That is, the relative increase in your damage is actually ~10%. Going back to the previous paragraph, a ~10% increase is small enough such that you can roll low and see a number that resembles a pre-buff attack that rolled high.