It shouldn't be much different, probably the seed just has a poor germination rate, which is a common thing with allium seed. Alliums will germinate over a very wide range of temperatures, just slower when cool. I'd plant more to get the number of plants you want.

Bad seeds. Either old or not matured.
Otherwise there is no other logical reson.
tasting seeds:
Lets say a gentle wind is blowing. Spread a sheet and take handfull of seeds and gradually drop them from a 2 feet height. The wind will blow away bad seed further from the perpendicular(underneath your hand). Only viablle seeds will be close to perpendicular. Pick those and throw away the rest.

I just got some leek seeds from the USDA a couple of weeks ago and their germination test says %100 percent germination rate.

I planted 12 in a starter cells and all twelve have germinated as of today. The leeks I planted were labeled pearl onions - Allium ampeloprasum by the USDA, but my readings says that the true pearl onions should be Allium porrum var. sectivum. Hopefully they will be "true pearl onions".

I planted them indoors in OSH premium potting mix on Jan 26th.

I also planted chives on the 27th in regular garden dirt with a little bit of OSH mixed in. I left that outdoors until February 4th. On the 4th I moved them indoors. None have germinated yet. The seeds source was American Seed Company packed for 2010 growing season.

I have also planted garlic chives seeds outdoors directly sown in the dirt outdoors their germination rate is probably less than 30%. Those were planted in November of 09. These seeds I collected myself from my own garden.

The outdoor highs right now about 61 degrees and the lows are about 50 degrees.

Hope this data helps.

I started leeks and a couple types of yellow onions indoors a couple weeks ago. The leeks seemed to germinate at a much lower rate than the onions. I started all of the on a heat pad and then moved them under lights without a heat pad.

After awhile I decided to move the leeks from the lights to the heat pad at night. After just a few nights of this heat treatment many more of the leeks seeds have germinated. My original low impression of the vitality of the leek seeds was misplaced. They just needed heat longer than the onions did.

The rest of my leeks sprouted around 2/7, or about two weeks after sowing. It looks like the germination rate is comparable with the others; they just took an extra week to come up.
It's kind of interesting there were two distinct 'waves' of germination.