This bugs me in general with many of the published modules. There's a bunch of encounters that are so close together, it really makes no sense for the guards or critters or whatever in the next room to just twiddle their thumbs waiting for the PC's to advance. But, if you merge encounters, they can get much, much more difficult (despite being the same XP). It's as is there's a gentlemens agreement to pretend the other group doesn't exist. The party avoids peeking around the corner since then they can get embroiled in multiple encounters, and the monsters pretend not to hear anything.
But that behaviour just makes no sense for either party. The PC's would want to know when to retreat if necessary; without a DM promising to hold back the hordes, they don't have a clue what's coming around the corner - ignoring the chance of being interrupted might be a death sentence.
And for the "monsters" it makes even less sense. In some of those modules, the PC's need to make several short rest of even long rests; there's just no way a sane garrison wouldn't know it was being attacked by then. Yet, they persist in attacking in the worst possible fashion, giving the PC's the initiative and only attacking in small manageable chunks.
It bugs me that there's no good in-game explanation for these things.
Published modules shouldn't bunch encounters together like that without some regard for why they're actually several encounters.
This is a good point, and why, even though I use the published modules, I take a LOT of time to look at them and rework stuff as necessary. If it would make sense for the enemy to do something, I either want them to do it, or rewrite the module so that it doesn't make sense for them to do it.
In regards to, say, the Chamber of Eyes fight in Thunderspire, the bandits in one room did not come out to assist the other bandits, but they instead made their own area much more defensible (overturned tables, blocked areas, etc). Then they and the slavers in the adjacent room planned a fighting retreat. Whichever room got entered, the bandits and slavers would fight them there, and retreat into the next room as they became bloodied.
This created a great situation where people fell back as they got hurt, even thought they weren't defeated.
The only problem was the PCs, who found the defended position of their enemies frustrating, decided to (DURING THE FIGHT) pull back half of their forces and scout an adjacent room. Of course, that room was where the remaining enemies and bloodied enemies had retreated!
I don't make any 'gentleman's' agreements for my enemies. On the other hand, my enemies also don't get any metagame knowledge of character levels either. The duergar don't guard the one door with every single man, armed with crossbows, though that would be effective, because SO FAR, no band of 5 nutjobs has ever been able to breach the gate when it was guarded by 5 orcs.
I do agree, though, most emphatically, with the desire that the published adventures would be written with a bit more care, so that highly untenable combats would be avoided through good construction, rather than 'dumb monster' tactics or careful rewriting.