The first comment is right. In every case, when dealing with finities, you lose energy by synthesizing fuel.
If you think of a balloon as a storage mechanism, then the energy lost is from:
- the sounds the balloon makes as it expands
- the stresses exerted upon the balloon itself that will eventually cause it to wear out and pop if repeatedly deflated and inflated to the same pressure
- the heat (although miniscule in this case, still very real) generated by the action of the air molecules against each other and against the balloon
- possible other avenues I haven't thought of or that we don't even know about
To put it plainly, every kind of fuel is trapped energy, and the trapping process always leaks energy in another form. The efficiency of that process is dependent both on the natural circumstances and on our own technical knowledge, but in an ideal case peaks around 95%
By comparison, our current energy storage mechanisms have a round-trip efficiency that ranges from 45% (hydrogen) to up to 95% (lithium-ion battery that isn't filled to it's full capacity) real-world more like 87% (Tesla Powerwall). For fuel-oil down-conversion to gasoline, there's around 85% efficiency, but the energy doesn't have to be generated, as we're using found energy.
The real driver of these technologies is convenience and applicabilitiy to a particular purpose. Even for rockets, we use the most convenient and applicable fuels for that specific purpose. Hydrogen is generated at 60-90% efficiency, and lots of energy is lost in the process of burning the fuel to launch the rocket to space. But, it gets the job done. For cars, now that electricity can be stored in energy densities that parallel gasoline (not quite there, but close enough) we're switching to electric, because it's easier. The applicability of the fuel for the specific purpose and the difficulty of handling the material matter a lot.
The crux of the issue is that the efficiency of creation can't realistically be separated from the difficulty of handling the material itself. And dealing with antimatter is HARD. If it's charged, or if it's ferrous antimatter, you can contain it with magnets. If not, you can.. ..what.. ..push it around? Nope. If it's ferrous, it's probably solid. How exactly do you plan to use that again? ..or wait.. ..to generate it? How are you going to get an iron antimatter block, even if you can generate iron antimatter molecules? ..melt it with lasers while containing it in a magnetic field, I guess, but.. ..eesh.
All of this boils down to the simple extreme probability that long before we have the ability to readily handle antimatter, we will have developed extremely efficient nuclear fusion or fission options. Bear in mind, the reality you're dealing with on energy density is:
1 part antimatter + 1 part matter = 2 parts energy
1 part matter + 1 part matter via fusion or fission = 2 parts energy
edit: that is to say, any talk of 'free' energy should consider that all nuclear, including antimatter, will have a large overhead, but are the harvesting of free energy. The advances in nuclear at this point will come in cleanness and in controllability, such as with molten salt reactors and laser fission. These processes will be convenient from our current standpoint, and will be improved as time goes on. /edit
..and the second is easier to work with, and therefore easier to generate efficient processes for. Again, we can count on the idea that for a readily-accessible, handlable technology with good natural conditions, we'll peak around 95% efficiency, round-trip. ..and that'll be long after the thing was used ubiquitously.