Tyre Rubber Classifications
Even though I don't have specific knowledge on road bike tires, nor specific numeric durometer designations, I have some expertise on MTB tires. The given specification numbers mean that the bike has a slightly harder rubber compound on the rear tire.

The reason for having different tires front and rear is that the dynamic loads that each tire has to cope with are different, all depending on bike geometry, rider technique and skill level and type / usage of the bike. 

The rear tire copes mainly with traction load, that is pushing the bike forward, also side load from turning but less than the front tire and a little fraction of the braking forces. Also during acceleration and rolling on flat, it bears a bigger fraction of the rider + bike weight.

Front tire on the other hand bears with almost all the braking force, specially on road bikes. It also copes with most of the force needed to steer the bike, which means greater side loads than the rear tire. Also, in strong braking the front tire bears with a bigger fraction of the weight.

When a forward moving wheeled vehicle brakes, a weight transfer occurs from the static center of mass towards the front of the vehicle. This is easily observed on cars, motorcycles and full suspension bicycles. That's why front tire works harder during braking, because the rear wheel has lesser downward force applied to it, so it is very limited on the friction it can create.

In the particular case shown on the question, the bike has a slightly harder rubber compound on the rear tire. This means that it will last a little longer and will create a little less rolling resistance (Compared to a tire with the same structure but using the softer compound and keeping all other variables equal). Conversely, the front tire, being softer will have more grip and will do a better job braking and steering.

In my opinion that makes sense because on a road bike, you spend more effort pedaling on flat or climbing, and during such events, the rear wheel has a grater fraction of the weight applied to, that is, the bike has less rolling resistance than if it had both tires made of the softer compound, but a little more grip than with both tires made of the harder compound. It's like trying to get the best of both worlds.

In MTB where my experience comes from, different threads is common practice. It is usual to pick tires independently, even different brands. Some manufacturers even have front and rear specific tires with the same model name. Others design a rolling direction for front use and the other for rear use. 

On my particular case I have bikes with different tires front and rear but also bikes with the same tire model. While comparing wear on different tires is very difficult, on the bikes I have with the same tire front and rear, there are different wear patterns.

On and old road bike that I have with cheap identical tires front and rear, I use a little more pressure on the rear tire, for the reasons explained 3 paragraphs before.