This may be a glitch or nerf.

[Olorin401]

I'm assuming the OP is casting the same spell twice in a row, in which case they should know that for any spell if you then cast another spell of the same element there will be a extra chance of resistance to that element. Likewise, if you use a spell that is weak to the spell you just used, that resistance will be stronger; if you use a spell that is STRONG to the spell just used you will gain an accuracy bonus. This is not a characteristic of the mob, it's the nature of magic based on the elemental wheel.

This resistance is even higher against NMs, because of cumulative magical resistance that SE added a while back. When you use ANY spell against a NM, its resistance to ALL magic of the same TYPE (Elemental, Enfeebling, Dark, etc.) AND that particular element goes up exponentially. This is part of the reason some folks will cast Drown or Flood 2 on certain NMs - it may make their magical resistance higher, but it will greatly lower their resistance to Lightning-based magic and overrides the cumulative resistance. Skillchains also lower magical resistance to its corresponding elements, which is partly where the bonus on a magic burst comes from.

[Armando]

Unless we're talking about NMs, you can cast the same spell over and over and over and over until the mob dies and resistance shouldn't change. Even then, for NMs, it's not a change in resist rate, but an increase in damage reduction.

I still say the OP changed his INT accidentally.

Sabaron, where did you get that formula? I've been curious about the different resist levels for a while.

[Karinya]

What about a gradually fading buff (etude?) or debuff (burn? sound blast? absorb-INT?) that affects your or your target's INT by a few points between the time of first and second casts?

Also, I think Olorin is wrong and only a few spells change target resistance, chiefly ninjutsu. I guess the best way to test it would be to chainspell and cast the same spell a LOT of times in a row...

[Sabaron]

Unless we're talking about NMs, you can cast the same spell over and over and over and over until the mob dies and resistance shouldn't change. Even then, for NMs, it's not a change in resist rate, but an increase in damage reduction.

I still say the OP changed his INT accidentally.

Sabaron, where did you get that formula? I've been curious about the different resist levels for a while.

I inferred it from the original link that I gave--e.g. off the Wiki:

Calculating Magic Damage - FFXIclopedia - a Wikia Gaming wiki

Wherein, it says:

Resist

resist = 1, 1/2, 1/4, 1/8, or 1/16

or, in formula, 2^n where n is the resist tier. I've also seen it before on Alla.. I believe in a post about directional casting (please, don't go telling people resists are based on the direction you're facing when casting spells -_-, I'm not advocating the post).

I also agree on the INT changing thing. Sounds exactly like a ring swap.
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I just thought of another possibility..

If he was eating Rolanberry Pie (INT+2) and it wore off between casts, that would cause dINT to go down by 2 points.