I don't know what kind of grass is there, but is it possible that they planted a grass that likes more shade?

If not given sufficient water, trees can often steal what would otherwise be given to the lawn.
Some trees, such as maple, are downright criminals as far as water thievery.
Most trees have their roots within 18" - 24" of the surface so their taking of water away from grass roots is quite easy to see happening.

If you wish to increase the water holding capacity of your lawn try this. In the spring, scatter 1/2" - 1" of good decent compost or topsoil over your lawn while the grass is just at the early stage of growing.
Then repeat this in the fall--about October. This giving t the lawn organic matter will encourage the soil there to hold on to more moisture and through winter will aid the grass from winter die-back.
If you repeat this every spring....and if you feel its doing some good, every spring and fall.
A fertilizing program should be initiated. Whether you give your lawn fall fertilizer is up to you but sometimes lawns do much better when this is done and it saves the amount of fertilizer given it in the spring.

During hot spells, such as like the one we're experiencing right now, let your lawn grow more--protecting against moisture loss from sun and when you mow, leave the clippings as much as possible on the lawn.
Allowing grass to grow --say to 3" - 4" high and then cutting only 1" - 1 1/2", shades weed seeds and thus can prevent some of the weeds from growing.

To be honest, I haven't watered my lawn once this year. The grass around the house is great despite this.

goren,

The house was built in a field 10 years ago. There are only 2 small trees in the front and one in the back.

Therein lies your problem. The shadier areas by the house don't use as much water, whereas the full sun areas in the yard need more. Also if you don't have gutters then those areas get more water when it rains. I suppose there could be some soil differences near the house, perhaps it holds water better, but I doubt it.