Although the previous answer by @ivonesh is totally correct and has awesome links (+1 for that), your battery might be showing signs of early wear.
It's not a big deal, and it could still make it another 800 cycles and only lose another 4% of capacity, but do keep an eye on it. It's certainly within spec right now, but the loss in capacity seems to be higher than expected for 17 months and 176 cycles.
If you have coverage for three years, you might take it in before that ends to see where you stand in another 19 months. Also - if you can keep it cool (well below 85°F on average) that will reduce the losses all Lithium based batteries have due to time and temperature.
Keep using it, just be aware that once it starts to go, the capacity tends to fall off more drastically and not slowly like in the first years. Also, there's lots of good use still in a battery once it's below 80% full charge capacity - so that's not anything other than a sign it's aging and it's also the line in the sand where apple expects will cover a battery due to premature loss of capacity.
All batteries degrade over time, but bad batteries degrade much faster or fail before they can provide the 1000 cycles they were designed to last.