Assuming you're not trolling, it's because this AI is using a heuristic approach rather than a guaranteed correct approach.

Heuristics rely on simplified rules which don't accurately model the system on which they're acting 100% of the time. Good heuristics can come close to 100%, however.

But why? Glad you asked!

A 100% correct solution would be to write an algorithm which enumerates all of the possible moves as a decision tree, and walks the decision tree to find the correct answer.

However, given that there are four possible moves the user can make (up, down, left, right) and an upper bound of 32 possible moves the computer can make (computer places "2" or "4" anywhere in a 4x4 grid), each level of the tree could require up to 128 times the number of computations that it took to compute the previous level of the tree.

For example, calculating the first turn is on the order of 128 calculations. Calculating the second move is on the order of 128^2 calculations. Calculating the third is 128^3, an so on. By order of magnitude, how many moves do you think it takes on average to get to 2048? 10^3? Even if we were being really optimistic, maybe it's 10^2. In that case, you'd have to perform somewhere around 128^100 computations in order to solve the game perfectly every time.

Incidentally, python tells me that'd be

5260135901548373507240989882880128665550339802823173859498280903068732154297080822113666536277588451226982968856178217713019432250183803863127814770651880849955223671128444598191663757884322717271293251735781376

calculations.

Or for fun, this is 128^1000:

16216967556622020264666650854783770951911124303637432562359820841515270231627023529870802378794460004651996019099530984538652557892546513204107022110253564658647431585227076599373340842842722420012281878260072931082617043194484266392077784125099996860169436006660011209817579296678781962552377006552947572566780558092938446272186402161088626008160971328747492043520874011018626908423275017246052311293955235059054544214554772509509096507889478094683592939574112569473438619121529684847434440674120417402088754037186942170155022073539838122429925874353753616104159343594557666561701790904172597025336526662682021808493892812699709528570890696375575414344876088248369941993802415197514510125127043829087280919538476302857811854024099958895964192277601255360491156240349994714416090573084242931396211995367937301294479560024833357073899839202991032234659803895306904298017400980173252106913079712420169633972302183530075897845195258485537108858195631737000743805167411189134617501484521767984296782842287373127422122022517597535994839257029877907706355334790244935435386660512591079567291431216297788784818552292819654176600980398997991681404749384215743515802603811510682864067897304838292203460427757655073776567547507027144662263487685709621261074762705203049488907208978593689047063428548531668665657327174660658185609066484950801276175461457216176955575199211750751406777510449672859082255854777144724233490076402632176089211355256124119453870268029904400183858505767193696897593661213568888386800238409325673807775018914703049621509969838539752071549396339237202875920415172949370790977853625108320092839604807237954887069546621688044652112493076290091990717742355039135117441532973747930089955830518884135334798464113680004999403737245600354288112326328218661131064550772899229969469156018580839820741704606832124388152026099584696588161375826382921029547343888832163627122302921229795384868355483535710603407789177417026363656202726955437517780741313455101810009468809407811220573803353711246329589162370895804762245950918253016369092362406714116443316561598280583720783439888562390892028440902553829376

So throwing a few assumptions in there isn't a bad idea...