The spacing on the tag is to allow the plant room to grow and it's based on how big the plants will get when they're full grown. If you plant according to the spacing on the tag, then when the plants get full grown the space in between should be filled in, but the plants won't be growing into each other.
In the pictures in magazines, the plants have been allowed to grow, so they've filled in the space in between already which is why they look closer. But if you measured the distance from the center of one plant to the center of another, you would probably find it's fairly close to what the plant tag told you. Or, if things are planted specifically for the purposes of posing for a picture they may plant things too close to begin with
If you want a filled in look sooner and are willing to trim things more or pull some plants out once everything's full grown, you can plant things closer than what the plant tag says and your garden will look full sooner. If you're planting shrubs & trees and things that are going to be more permanent fixtures in the garden then I'd follow the spacing on the tags (or only cheat a little bit). If you don't then it creates a lot more pruning for you later. But if it's annuals and you want things to look more filled in then it doesn't hurt to plant things closer together.
pix 1:West border
pix 2:East border
pix 3:from within the border, here you can see gaps between plants.. Most gaps are there for plants to grow together, or "Knit together" (plants touching when full grown so the ground is not seen)
pix 4: here in this area of the garden, it's a few years older than the previous pix, notice how close the plants are now
pix 5: Here is a vinette, the flowers were planted (mostly from 21/2" pots, grown from seed) with the proper space between plants to acheive a filled in look from the matured plants. Does that help to answer your question?
pix is a section of the border with perennial Hesperis matronalis (Sweet Dames Rocket). I origonally planted about 50 21/4" pots. Then I let them reseed and this is the result, a mass planting with hundreds of plants..and the fragrance is spectacular!!!
pix 2: is the origonal planting, second year, now you can see how much things have filled in, the above pix is probably year four.
This area has become rather crowed now and will be heavily edited (nice way of saying pulled out and thrown on the compost pile). Most of these plants are not spaced properly due to the reseeding that was allowed, but does give you an idea of what ecrane was talking about (planting closely).
To give you an idea of the size of my 2 borders (pix 1&2 above) it's 45ft wideX100ft long. The hesperis are almost 4ft tall. Good Luck, Kathy.