Can slimes spawn in chunks surrounding slimechunks
Solution 1:
Slimes can only spawn in chunks where they are allowed. Why are there slimes in chunks that shouldn't have them?
Slimes move around. Chances are this is what happens. They will bounce (ooze? slide?) to different chunks over time. This can include falling from higher heights.
You used or read the slime chunk calculator wrong. Maybe you punched in the seed wrong or chose the wrong map. Maybe you misread your map? Double check you if you needed to have a "-" (negative sign) on your coordinates. Double check your calculator.
This may or may not apply to your spawning pads but slimes can spawn at any light level, unlike other mobs like skeletons, zombies, etc.
For reference: Minecraft Wiki - Slime http://www.minecraftwiki.net/wiki/Slime
Solution 2:
Maybe it is due to how the spawning mechanism works in general. See the wiki:
"If the pack location is suitable, 12 attempts are made to spawn up to 4 mobs (8 for Wolves, 1 for Ghasts) within a 41x1x41 area centered at that block (that's a 41x41 square that is one block high). Mobs will spawn with the lowest part of their body inside this area. For each spawn attempt, a block location within the pack area is chosen at random. Though the pack area extends 21 blocks out from the center, the random location is heavily skewed toward the center of the pack. Approximately 85% of spawns will be within 5 blocks of the pack center, and 99% within 10 blocks of the center."
Assuming it chooses an air block at the edge of your eligible chunk as the spawn center, this might cause it to spawn outside the chunk.