First of all, there is nothing like a one fifth gravity beer. 1/5 = 0.2, which means nothing in terms of fermentation.
Your question must be answered in three parts, the first part being what are you measuring, how it is done, and what the results say, and the second part is about the work the yeast does, and the third about fermentability.
The hydrometer is your basic tool, which measures the density of a sugary water solution. When you measure 1.000, then you have pure water. When you measure in a solution with sugar then this number increases.
Before fermenting (after the boil), you measure the original gravity (OG). After the fermentation you measure the final gravity (FG). From this, the apparent attenuation is derived by the formula (OG - FG) / (OG - 1).
This is called the apparent attenuation. The reason for this is that the solution now also contains alcohol in addition to water and sugar. This has as a consequence that the liquid density (alcohol + water) is now less than that of water, and your hydrometer actually sinks lower.
So, to answer already part of your question: "spirit indication" is not taken into consideration, nor are other solids in solution.
Now for the second part. Wort is actually a very complex solution of all kinds of different sugars, in different kinds of concentrations depending upon the different malts used.
The attenuation of yeasts is influenced by two parameters: the number of different sugars that yeast can ferment and their flocculation, the rate of dropping to the bottom of the fermentation vessel.
The attenuation given by the producer of the yeast mostly indicates if yeasts will consume more differentiated sugars, and are, as a result, capable of attaining a higher attenuation.
However, this is also influenced by the flocculation. When the yeasts drop out of the beer, there will be less fermentation as a result. If it would be possible to regularly rouse the yeast, then fermentation would go on (in England they used to do this, certain English yeast strains drop out rather fast, and the brewers would then roll the casks where the beer was kept to get better attenuation).
You can influence the fermentability of your wort by playing with the mash temperatures, but there are no hard and fast rules.
There are two enzymes in the mash, alfa-amylase and beta-amylase, which convert starches to complex and simple sugars. Alfa-amylase works best on starches, but its optimal temperature is around 70° C. Beta-amylase works best in converting complex sugars to simple sugars, but it works best at 64° C. These enzymes don't last in the mash, because they are denatured by the temperature of the mash. This takes its time of course, but the higher the temperature, the faster they are denatured.
What does this mean for fermentability? A rule of thumb is that mashing for a long time at 64° C will create a very fermentable wort, while mashing at higher temperatures will decrease the fermentability.
However, it's not possible to put figures on this, i.e. if you mash at this temperature, you will get this kind of fermentability is not something that can be put into tables.
My experience is that most yeasts do ferment wort in the ranges given by the producer.
So, if the only thing you have is a particularly attenuating yeast, then mash at higher temperature, e.g. 67° C or 68° C, and see what happens to your FG.