The theory of sweat patterns can be very confusing... The idea that a dry area is caused by high pressure has come about because a lot of horse's are ridden in saddles that are very tight in the front and putting a lot of pressure in this area.  Sometimes the pressure can close off the blood vessels and even damage nerves if very high.  This is why it rightly worries people.
However, unless you are using a BALANCE saddle in a very incorrect or strange way, it is almost impossible to get high pressures underneath it.
There are several reasons why people see uneven sweat patterns and ‘dry spots’ in the wither area or in other areas under the saddle after riding:
Pressure from the points of the tree. This a very common problem with conventionally fitted saddles, but it is very rare with BALANCE saddles unless the saddle is not balanced correctly (for example, if the saddle is slightly tipping forward and/or the rider is very braced into their stirrups).Previous pressure from a tight saddle has caused the area under the points of the tree to become 'shut down' and the sweat glands stop working correctly in that area. This often improves gradually in a less tight saddle.If a BALANCE saddle is very generous in width, then there is room for sweat to evaporate from under the points of the tree as it produced (like it is on the rest of the body that is not covered). This area does not come into constant contact with the saddle and so there is room for the air to circulate through the fibres of the wool fleece here. After riding, the rider will then find 'dry spots' under the points of the tree.We sometimes see a drier area in the middle of the back when the horse is not quite engaged up into the saddle enough, so the saddle is, in effect bridging very slightly. Quite a lot of people use this symptom to identify how well their horse is moving and when they get even sweat patterns it identifies a really good training session where the back has been lifted and the pressures more even.If a horse and/or rider is habitually not straight it can create uneven pressure left-to-right on the saddle, which in turn can create uneven sweat patterns when comparing left and right-hand sides of the horse after riding.
To summarize:
A lot of horse owners have been trained to believe that a dry looking area always means high pressure, but in fact, it can mean quite the reverse. We always say that a rider should not ignore a change in sweat patterns, but also not assume that they are necessarily bad. It is a good idea to get into the habit of palpating the horse's back gently, at least one hour after riding to assess if the horse is comfortable, and always pay attention to the way the horse is moving under saddle.