why Target is very different between ethpool.org , pooleum , on my local machine?

I've found something wondering things.
I use ethminer with pooleum.com samples.

Target is : 00000002dd.....

i 23:06:30|main Seedhash: 356e5a2cc1eba076e650ac7473fccc37952b46bc2e419a200cec0c451dce2336
i 23:06:30|main Target: 00000002dd01fc067918c87bb22ae2da831babaff47cc1f55b39398a682631f0
i 23:06:30|main Ethash: 0000000240a22108589f36e9daee50fb6961f1917944cfada5615de0ff90573d
....
miner 23:14:44|main Got work package:
miner 23:14:44|main Header-hash: b52fca3d202c705460b4dbebca5c8de3308393861a7b048474418ef4cc329f26
miner 23:14:44|main Seedhash: 356e5a2cc1eba076e650ac7473fccc37952b46bc2e419a200cec0c451dce2336
miner 23:14:44|main Target: 00000002dd01fc067918c87bb22ae2da831babaff47cc1f55b39398a682631f0
i 23:14:44|gpuminer0 workLoop 1 #356e5a2c??#356e5a2c?


ethminer with ethpool.org

Target is : 00000001b7......

miner 23:14:59|ethminer Got work package:
miner 23:14:59|ethminer Header-hash: 3052cf5039c8e7e3d87bbf56b5fed78a3dae18727e1b3828fbeac38513346539
miner 23:14:59|ethminer Seedhash: 356e5a2cc1eba076e650ac7473fccc37952b46bc2e419a200cec0c451dce2336
miner 23:14:59|ethminer Target: 00000001b7cdfd9d7bdbab7d6ae6881cb5109a365f7e0df99d2255b971b0845d

in case of my linux machine's geth worknode with Farm

Target is : 00000000003ecf.....
miner 23:18:14|ethminer Header-hash: 47b76a369027d55f73ac69adacb5c23a6192b2cf6a449401a4f2e22cdcbdcb02
miner 23:18:14|ethminer Seedhash: 356e5a2cc1eba076e650ac7473fccc37952b46bc2e419a200cec0c451dce2336
miner 23:18:14|ethminer Target: 00000000003ecf4a534d77a01568e9eafbf62edcc7dd3e7097932440d5053afe
miner 23:18:15|ethminer Mining on PoWhash #47b76a36… : 89800 H/s = 44900 hashes / 0.5 s
miner 23:18:15

What's problem? and why so big differences?