You can aim for 60 and still not necessarily achieve a locked or consistent 60. You have a performance budget for say X section of the game for A, B, C, etc. things (polygons, textures, animations, AI, etc.). Artists are always going to want to try to push that boundary as far as they can and you'll run into a situation where a combination of all of those "things" cause a sub 60 framerate due to the interaction of all of those "things" even if the each of those "things" is within the performance budget of that section of the game.
COD on PS3/X360 often had PS3 unable to stay at 60 FPS even though COD was only targeting 60 FPS with no 30 FPS option.
Sometimes, you have time to then optimize every section that runs into a performance deficit despite technically being within the budget. Sometimes you don't because the deadline for shipping is getting closer and you can't delay the game or can't add further delays to the game.
Having a target is just that a target. Either you play it conservatively with your graphics and artists to maintain that target, or you try to push it as close to the target as you can and you end up with performance not locked to your target (to a greater or lesser degree). And it's not just 60 FPS targets, 30 FPS targets also often suffer drops. Except if your target is 30 FPS, any drops are far worse than dropping from 60 if they are a similar scale. IE dropping from 30 to 25 is going to be more immediately noticeable to more people than dropping from 60 to 50 (still noticeable to some people, but others will never notice unless they see a frametime graph).
Regards,
SB